Add test that _rm_lock_files actually removes lock files

This test creates a real `.lock` file in the repository's `.git/`
directory and asserts that `_rm_lock_files()` removes it. As of this
commit (i.e. without the follow-up fix), the test fails: `_rm_lock_files`
has been a silent no-op for several years because of a stale path
expression in `_tc_lock_fpaths` just above it.

The follow-up commit ("Fix stale path in `_tc_lock_fpaths`") corrects
the path; with that fix in place this test passes.

This is added test-first (failing-then-passing) so the regression
guard is recorded as a real demonstration of the bug, the way
`test/test_fixture_health.py` was added test-first earlier this year
for the Cygwin safe.directory issue.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliahKagan

test/test_repo.py

@@ -62,6 +62,41 @@ def _rm_lock_files():
         rmfile(lfp)
 
 
+def test_rm_lock_files_actually_removes_lock_files():
+    """Verify ``_rm_lock_files`` actually finds and removes ``.lock`` files.
+
+    Regression test for a path bug introduced when the tests directory was
+    moved from ``<repo>/git/test/`` to ``<repo>/test/`` in commit 0374d7cf
+    (2020-07-12, "tests: move to root dir"; see also d5f0d487 for the
+    related revert that landed alongside it). ``_tc_lock_fpaths`` above
+    was originally written for a file two directory levels deep
+    (``../../.git/...``); after the move it should have become
+    ``../.git/...``. The miscount caused :func:`glob.glob` to look in
+    ``<parent-of-repo>/.git/``, which usually does not exist, so the
+    helper has been a silent no-op for several years.
+
+    This test depends on ``__file__`` resolving to the real
+    ``<repo>/test/test_repo.py`` at runtime (i.e. running the tests from
+    the source tree, as CI does). When ``<repo>/.git`` is not a directory
+    -- e.g. in a linked worktree where it is a gitfile -- the test is
+    skipped, since the cleanup helper isn't meaningful in that context.
+    """
+    repo_dot_git = osp.join(osp.dirname(osp.dirname(osp.abspath(__file__))), ".git")
+    if not osp.isdir(repo_dot_git):
+        pytest.skip(f"{repo_dot_git} is not a directory; cleanup helper isn't meaningful here")
+
+    canary = osp.join(repo_dot_git, "_test_rm_lock_files_canary.lock")
+    open(canary, "w").close()
+    try:
+        _rm_lock_files()
+        assert not osp.exists(canary), (
+            f"_rm_lock_files() left {canary} in place; _tc_lock_fpaths is probably pointing outside the repo's .git"
+        )
+    finally:
+        if osp.exists(canary):
+            os.unlink(canary)
+
+
 class TestRepo(TestBase):
     def setUp(self):
         _rm_lock_files()