[tests/functional/test_cache] Use os.path.join() instead of hard-coding the path separator.
[tests/functional/test_cache] Use os.path.join() instead of hard-coding the path separator.

pfmoore/pip,pradyunsg/pip,sbidoul/pip,sbidoul/pip,pypa/pip,pradyunsg/pip,pfmoore/pip,pypa/pip