tests: Ensure patched `os.getcwd()` is an absolute path

`os.getcwd()` returns an absolute path; thus, the patched paths should
be absolute as well.

This is an incremental change towards removing the `ConfigFileFinder`
attributes `.parent` and `.tail` to be localized to
`.generate_possible_local_files()`.  Without this, the tests fail when
moving the patching because `os.path.abspath()` calls `os.getcwd()`,
expecting `os.getcwd()` to be an absolute path.