[3.12] gh-108927: Fix removing testing modules from sys.modules (GH-108952) (ПР-112711)

It breaks import machinery if the test module has submodules used in
other tests.
(cherry picked from commit e08b70fab1fbc45fa498020aac522ae1d5da6136)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>