Test that plugin hooks are unit tested (xfail)

This currently fails using xfail. Closes 771.

tests/test_plugins.py

@@ -7,7 +7,7 @@
     TestClient as _TestClient,
 )  # noqa
 from datasette.app import Datasette
-from datasette.plugins import get_plugins, DEFAULT_PLUGINS
+from datasette.plugins import get_plugins, DEFAULT_PLUGINS, pm
 from datasette.utils import sqlite3
 import base64
 import json
@@ -20,6 +20,20 @@
 import urllib
 
 
+@pytest.mark.xfail
+@pytest.mark.parametrize(
+    "plugin_hook", [name for name in dir(pm.hook) if not name.startswith("_")]
+)
+def test_plugin_hooks_have_tests(plugin_hook):
+    "Every plugin hook should be referenced in this test module"
+    tests_in_this_module = [t for t in globals().keys() if t.startswith("test_")]
+    ok = False
+    for test in tests_in_this_module:
+        if plugin_hook in test:
+            ok = True
+    assert ok, "Plugin hook is missing tests: {}".format(plugin_hook)
+
+
 def test_plugins_dir_plugin_prepare_connection(app_client):
     response = app_client.get(
         "/fixtures.json?sql=select+convert_units(100%2C+'m'%2C+'ft')"