Problem

Several Python tests that are not dedicated tiered-storage tests (alter, schema, excl, export, import11) still import TieredConfigMixin / gen_tiered_storage_sources and add a dir_store scenario. Those mixins must be stripped before helper_tiered.py is deleted.

Change

Keep the tests. Remove only the tiered mixin, dir_store scenarios, and flush_tier / *.wtobj cases. Preserve import and backup:export coverage. Do not delete helper_tiered.py in this ticket (that is the dedicated-test ticket). Do not change src/.

Test

Run the altered Python tests (test_alter_, test_schema_, test_excl, test_export01, test_import11) without the tiered hook.