[TEST] Moved wipe* methods, randomIndexTemplate & ensureEstimatedStats from ElasticsearchIntegrationTest to TestCluster

This is the first to make it possible to have a different impl of TestCluster (e.g. based on an external cluster) that has the same methods but a different impl for them (e.g. it might use the REST API to do the same instead of the Java API)

Closes #5542