The OnceLock introduced in 5fbd49a80 for bucket_metadata_sys panics when
multiple test modules initialize their own ECStore against the shared
bootstrap context. Switch to Mutex<Option> so test fixtures can
reinitialize safely.
Extract shared gating test environment to gating_test_env.rs to avoid
duplicate ECStore setup across delete_objects_stat_gating_test and
put_prelookup_gating_test.
Fixes 8 test failures:
- capacity_dirty_scope_test (2 tests)
- delete_objects_stat_gating_test (3 tests)
- put_prelookup_gating_test (3 tests)
Zhengchao An
cefbe2ccc9