Adapt migration tests for shard merge

UPDATE: (Review all the files with :TODO SERVER-61231: and see if we could adapt that test to shard merge.)

Some multitenant migration tests do "banned operations" that the new shard merge protocol can't handle:

• Run concurrent migrations
• Restart the donor primary, or any recipient node
• Trigger a rollback on the donor or recipient replica set

Sometimes they do banned ops specifically to test whether they work with migrations. We know that shard merge will never support these ops, so we can disable these tests when featureFlagShardMerge is enabled. Eventually we'll delete these tests when shard merge becomes the only migration protocol.

Other tests happen to do banned ops and they also test other features, for which we don't want to lose coverage when featureFlagShardMerge is enabled. E.g. tenant_migration_external_keys_ttl.js does concurrent migrations but also tests a TTL index. Adapt these tests somehow: either don't do  banned ops, or do them only when featureFlagShardMerge is disabled, or make a copy of the test for shard merge and remove the banned ops.