Fix flaky test MirrorConnectorsIntegrationTest.testReplication

The test org.apache.kafka.connect.mirror.MirrorConnectorsIntegrationTest.testReplication is a flaky test for MirrorMaker 2.0. Its flakiness lies in the timing of when the connectors and tasks are started up. The fix for this would make it such that when the connectors are started up, to wait until the REST endpoint returns a positive number of tasks to be confident that we can start testing.