In this OplogApplierFails test, we use an OplogFetcherMock to feed an oplog entry that doesn't belong to the tenant to test that TenantOplogApplier fails the migration. When failing the migration, the main chain would call _cleanupOnDataSyncCompletion which would clean up and free the _donorOplogFetcher. But this could race with the test thread that calls oplogFetcher->receiveBatch which could end up accessing freed memory if the _donorOplogFetcher object is freed before the receiveBatch returns.

To fix this, we could use the hangBeforeTaskCompletion failpoint to hang the main chain before _cleanupOnDataSyncCompletion is run.