Exclude migration_coordinator_shutdown_in_critical_section.js test from the config stepdown suite

The migration_coordinator_shutdown_in_critical_section.js test expects a chunk migration to still complete in the absence of a primary on the donor shard after the _configsvrCommitChunkMigration command has been successfully run on the config server primary. A newly-elected config server primary will attempt to contact a primary on the donor shard and time out waiting at around the same interval the JavaScript stepdown thread causes a new config server primary to be elected. The periodicity with the JavaScript stepdown thread prevents the moveChunk command from ever returning successfully and so the migration_coordinator_shutdown_in_critical_section.js test should be tagged with does_not_support_stepdowns.