Fix windows appdirs tests

[xavfernandez]

[xavfernandez]

@pfmoore First commit gave lots of assert '/home/test/Library/Caches\\pip' == '/home/test/Library/Caches/pip'
cf https://ci.appveyor.com/project/xavfernandez/pip/build/1.0.7/job/4vhtq5lp0137eipy

My os.path monkeypatch seems to work but maybe it would be cleaner to simply @pytest.mark.skipif("sys.platform == 'win32'") ?

appdirs_tests seems to be fixed in https://ci.appveyor.com/project/xavfernandez/pip/build/1.0.10/job/w9s58d0nqnjspu1r

[pfmoore]

@xavfernandez Thanks for doing this!

I prefer not skipping tests on Windows, because that means Windows users can't confirm they didn't break Unix functionality (the same applies the other way round - I like having Windows tests that work on Unix as well, so monkeypatching ntpath in to test the Windows code path is good).

[xavfernandez]

Merging this. There are still 9 unit tests failing, cf https://ci.appveyor.com/project/xavfernandez/pip/build/1.0.13/job/3365jywp02iuanh9