[3.12] gh-59215: unittest: restore _top_level_dir at end of discovery (GH-15242) (GH-117508)

* gh-59215: unittest: restore _top_level_dir at end of discovery (GH-15242)
(cherry picked from commit fc5f68e58ecfbc8c452e1c2f33a2a53d3f2d7ea2)


Co-authored-by: Zackery Spytz <zspytz@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>