Test: AckTests.test*Warmer* - make sure at least one shard is started

The Put Warmer API executes the search encapsulated in the warmer before accepting it. This requires that at least one shard will be started. The tests used to use ensureGreen to check for that because of a publish timeout of 0 (needed to check the ack mechanism) that doesn't guarantee the shard is really started - just that the master has changed the CS to say so. This commit changes the ensureGreen to a the indexing of a single document.