In TenantMigrationRecipientServiceTest, we use insertTopOfOplog to add oplog entries to the mock replica set so that _getStartOpTimesFromDonor can get the startFetchingOptime in unittests. In unittest RecipientForgetMigration_WaitUntilStateDocInitialized, we are missing such step and so sometimes it would hit this uassert that the unittest doesn't expect if the interrupt from onReceiveRecipientForgetMigration comes after the instance runs _getStartOpTimesFromDonor.

To fix this, use insertTopOfOplog to insert a dummy oplog into the mock replica set like other tests do.