This is a continuation of SERVER-84709.

In sharding/resharding_stepdown_during_critical_section.js an intentional critical section timeout triggered by the test is overriden by a resharding retry which subsequently succeeds, causing the test to fail since it expects the critical-section-timeout error.

The cause for the spurious retry needs to be found and eliminated or debugged.

Solution: The resharding_stepdown_during_critical_section.js was replaced with a C++ unit test.