test: restore abspath after include-order tests - #473

I found that TestOrderOfIncludes replaces os.path.abspath with an identity function, but the inherited tearDown() is not invoked for this plain pytest class. The replacement therefore leaks into later tests: FileInfo("source.cc").FullName() returns "source.cc" instead of an absolute path.

Use the monkeypatch fixture to restore the original function after each include-order test. Add a FileInfo regression in a real working directory and a changelog entry. The include-order tests keep their existing mocked-path behavior; production code is unchanged.

Validation on Windows / Python 3.12.3:

• Before the fixture fix, the new path test passes alone, but running TestOrderOfIncludes followed by TestFileInfo gives 13 passed, 1 failed.
• After the fix, the same combination gives 14 passed.
• Reproduction command: python -m pytest --no-cov cpplint_unittest.py::TestOrderOfIncludes cpplint_unittest.py::TestFileInfo.
• Full suite: 232 passed, coverage 96.46% (required: 95%).
• Pylint and Mypy passed.
• Pre-commit checks on both changed files passed, including Ruff check/format and Codespell; git diff --check passed.
• CI run 34807125497 passed for this head combined with develop: Ubuntu, macOS, and Windows, each on Python 3.10 and 3.14. All six jobs ran 232 tests successfully, with 96.46% coverage, and passed Pylint and Mypy. The remote pre-commit check also passed.