Ignore hidden files (.testmondata) in test_pytest_collect_file

`test_pytest_collect_file` fails if you run the tests using `--testmon`,
because pytest-testmon will put its DB there as `.testmondata`.

testing/test_collection.py

@@ -150,6 +150,8 @@ class TestCollectPluginHookRelay:
         wascalled = []
         class Plugin:
             def pytest_collect_file(self, path, parent):
+                if not path.basename.startswith("."):
+                    # Ignore hidden files, e.g. .testmondata.
                     wascalled.append(path)
         testdir.makefile(".abc", "xyz")
         pytest.main([testdir.tmpdir], plugins=[Plugin()])