recover_multiple_migrations_on_stepup.js fails when executed in config server stepdown suite

The failing test expects a specific migration to fail when a step-down of the donor interrupts the sequence.

Such condition is generally true: nevertheless, when a step-down of the config server node follows the step-down of the donor, the failing migration may be re-attempted as part of the balancer recovery sequence and hence succeed.

The BF may be corrected by modifying such assert condition.