Removed wrong assertion that makes recoverWhileUnderLoadWithNodeShutdown fail.

As we are within an awaitBusy block, it doesn't make sense to have an assertion, since it would fail the test instead of waiting till the condition is verified (till timeout expires)
Improved test

Introduced ElasticsearchAssertions that check for failures in all response
Added comment to flush requests as we don't check for failures there
Attempt to remove Thread.sleep in testChangeInitialShardsRecovery in favour of awaitBusy block
Added awaitBusy block in testQuorumRecovery waiting for YELLOW. We could have GREEN after the close node but wa want to wait for the YELLOW state.