gh-107471: Fix Refleaks in test_import (gh-107569)

gh-107184 introduced a refleak in test_import.SubinterpImportTests (specifically test_singlephase_check_with_setting_and_override and test_single_init_extension_compat).  We fix it here by making sure _testsinglephase is removed from sys.modules whenever we clear the runtime's internal state for the module.

The underlying problem is strictly contained in the internal function _PyImport_ClearExtension() (AKA _testinternalcapi.clear_extension()), which is only used in tests.

(This also fixes an intermittent segfault introduced in the same place, in test_disallowed_reimport.)