test_import_sorting.py failures when run from outside the source tree

[akaihola]

Describe the bug
When pytest is run from outside the Darker source tree, some tests fail.

To Reproduce
Steps to reproduce the behavior:

1. Clone Darker: git clone https://github.com/akaihola/darker
2. In a fresh virtualenv, install the cloned Darker with test dependencies: pip install './darker[test]'
3. Run tests with pytest ./darker/src/darker/tests/test_import_sorting.py
4. Observe test failures

Expected behavior
No test failures

Screenshots

pytest ./darker/src/darker/tests/test_import_sorting.py
================================================ test session starts ================================================
platform linux -- Python 3.11.3, pytest-7.3.1, pluggy-1.0.0
rootdir: /tmp/darker
configfile: pytest.ini
plugins: darker-0.1.2, kwparametrize-0.0.3
collected 64 items                                                                                                  

darker/src/darker/tests/test_import_sorting.py ..............................................F.F............. [ 96%]
..                                                                                                            [100%]

===================================================== FAILURES ======================================================
___________________________________ test_build_isort_args[src0-None-None-expect0] ___________________________________

src = PosixPath('file.py'), config = None, line_length = None, expect = {'settings_path': '/tmp'}

    @pytest.mark.kwparametrize(
        dict(src=Path("file.py"), expect={"settings_path": "{cwd}"}),
        dict(
            config="myconfig.toml",
            expect={"settings_file": "myconfig.toml"},
        ),
        dict(line_length=42, expect={"settings_path": "{cwd}", "line_length": 42}),
        src=Path("file.py"),
        config=None,
        line_length=None,
    )
    def test_build_isort_args(src, config, line_length, expect):
        """``_build_isort_args`` returns correct arguments for isort"""
        result = darker.import_sorting._build_isort_args(src, config, line_length)
    
        if "settings_path" in expect:
            expect["settings_path"] = str(expect["settings_path"].format(cwd=Path.cwd()))
>       assert result == expect
E       AssertionError: assert {'settings_path': '/'} == {'settings_path': '/tmp'}
E         Differing items:
E         {'settings_path': '/'} != {'settings_path': '/tmp'}
E         Use -v to get more diff

darker/src/darker/tests/test_import_sorting.py:184: AssertionError
____________________________________ test_build_isort_args[src2-None-42-expect2] ____________________________________

src = PosixPath('file.py'), config = None, line_length = 42, expect = {'line_length': 42, 'settings_path': '/tmp'}

    @pytest.mark.kwparametrize(
        dict(src=Path("file.py"), expect={"settings_path": "{cwd}"}),
        dict(
            config="myconfig.toml",
            expect={"settings_file": "myconfig.toml"},
        ),
        dict(line_length=42, expect={"settings_path": "{cwd}", "line_length": 42}),
        src=Path("file.py"),
        config=None,
        line_length=None,
    )
    def test_build_isort_args(src, config, line_length, expect):
        """``_build_isort_args`` returns correct arguments for isort"""
        result = darker.import_sorting._build_isort_args(src, config, line_length)
    
        if "settings_path" in expect:
            expect["settings_path"] = str(expect["settings_path"].format(cwd=Path.cwd()))
>       assert result == expect
E       AssertionError: assert {'line_length...gs_path': '/'} == {'line_length...path': '/tmp'}
E         Omitting 1 identical items, use -vv to show
E         Differing items:
E         {'settings_path': '/'} != {'settings_path': '/tmp'}
E         Use -v to get more diff

darker/src/darker/tests/test_import_sorting.py:184: AssertionError
================================================= warnings summary ==================================================
../home/kaiant/.virtualenvs/py311/lib/python3.11/site-packages/_pytest/config/__init__.py:1302
  /home/kaiant/.virtualenvs/py311/lib/python3.11/site-packages/_pytest/config/__init__.py:1302: PytestConfigWarning: Unknown config option: show_capture
  
    self._warn_or_fail_if_strict(f"Unknown config option: {key}\n")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================== slowest 10 durations ================================================
0.02s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content2-expect2-\n-utf-8]
0.02s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content2-expect2-\r\n-utf-8]
0.02s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content4-expect4-\n-utf-8]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content2-expect2-\n-iso-8859-1]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content4-expect4-\n-iso-8859-1]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content1-expect1-\r\n-iso-8859-1]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content3-expect3-\n-iso-8859-1]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content3-expect3-\r\n-iso-8859-1]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content3-expect3-\n-utf-8]
0.01s call     src/darker/tests/test_import_sorting.py::test_apply_isort[content2-expect2-\r\n-iso-8859-1]
============================================== short test summary info ==============================================
FAILED darker/src/darker/tests/test_import_sorting.py::test_build_isort_args[src0-None-None-expect0] - AssertionError: assert {'settings_path': '/'} == {'settings_path': '/tmp'}
FAILED darker/src/darker/tests/test_import_sorting.py::test_build_isort_args[src2-None-42-expect2] - AssertionError: assert {'line_length...gs_path': '/'} == {'line_length...path': '/tmp'}
====================================== 2 failed, 62 passed, 1 warning in 0.76s ======================================

Environment (please complete the following information):

• OS: Linux / NixOS 21.11
• Python version: 3.11.3
• Darker version: 1.7.2