test(corekit): test_sentinel_exports_unit fails 5 tests in the corekit unittest leg #1332

Describe the bug
tests/corekit/test_sentinel_exports_unit.py fails 5 times when it runs inside the corekit unittest leg, but passes on its own under pytest. Three failures are in SentinelExportTests.test_star_import_hands_back_the_canonical_object and two in NoValueIsTheDocumentedFieldDefaultTests. The pattern matches #1065: an object imported at module level goes stale once an earlier module re-imports pcapkit, so assertIs against the sentinel fails.

To Reproduce
On main (11ae68ebf), python util/run_unittest_leg.py corekit reports 593 test(s), 5 failure(s), 0 error(s). I re-ran it.

Expected behavior
The module uses reimport_once_per_class and imports what it needs inside each test, so that it passes in any order. CI does not run a corekit leg, which is why this has gone unnoticed.

Found by the #1254 worker and its cross-review.