Fix test that leaks a chdir'd cwd across the test run (#17)

test_relative_symlink_to_common_file changed the process working
directory via os.chdir() but never restored it, leaving subsequent
tests running from a tmp_path fixture directory that gets deleted
after the test. Restore cwd in a finally block, matching the pattern
already used in test_pseudo_direntry.py.

Also checks off the now-satisfied xfail-marker item: no xfail markers
remain in the repo.

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

IMPROVEMENTS.md

@@ -158,11 +158,11 @@ Acceptance criteria:
 - [x] Run CI on pull requests as well as pushes.
 - [x] Add macOS CI because macOS is an advertised platform.
 - [x] Fix the Unix-socket path test that currently fails on a local macOS run.
-- [ ] Replace nonexistent issue references in `xfail` markers with real tracked
+- [x] Replace nonexistent issue references in `xfail` markers with real tracked
   work, then remove the markers as fixes land.
 - [ ] Add subprocess-based concurrency, signal, interruption, and fault tests.
 - [ ] Add filesystem-boundary and build/install smoke tests.
-- [ ] Fix tests that change global state or assert against the wrong path.
+- [x] Fix tests that change global state or assert against the wrong path.
 
 Acceptance criteria: