Migrate tests under tests/cluster to tests/unit/cluster to use the same framework all the cluster tests.

Test file migration:

• 03-failover-loop
• 04-resharding
• 07-replica-migration
• 12-replica-migration-2
• 12.1-replica-migration-3
• 28-cluster-shards

Usage Cleanup:

• runtest-cluster and it's access across build files and continuous integration (yml)