Unify temporary directory allocation strategy in tests

[KmoM88]

Following the upstream unification in colcon/colcon-core#729, this PR refactors the test suite to use the modern tmp_path fixture (which provides a pathlib.Path object) instead of mixing multiple temporary directory strategies.

Specifically, this package previously used:

• The Pytest tmpdir fixture (in test/test_environment_library_path.py)

This refactor:

1. Adds a test/conftest.py compatibility fixture for tmp_path (supporting older pytest versions, such as those on Enterprise Linux 8).
2. Updates test/test_environment_library_path.py to use tmp_path instead of tmpdir.
3. Cleans up an unused Path import and removes a redundant global spell_check_words_path declaration in test/test_spell_check.py to resolve pre-existing linter errors on modern environments.
4. Updates test/spell_check.words to include words introduced by the compatibility fixture (isdigit, itertools, linux, takewhile, tmpdir).

Reference

• Aligns with the testing strategy established in: colcon/colcon-core#729

Specifics

Unlike other submodules which used manual TemporaryDirectory blocks, colcon-library-path only used the tmpdir fixture.

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