Add integration tests for elasticsearch.sniff* config

[mshustov]

Setting elasticsearch.sniff* in the config file used to cause authentication problems #42224
The root problem has been fixed in #91276
We should add integration tests to make sure that code changes do not break cases when users configure elasticsearch.sniff* settings.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

Even if it would make sense to add such integration tests, we now have unit tests asserting that the generated client configuration is on the expected shape to avoid this issue (auth info at the top level of the config and not per node), and implementing those tests, in addition to mostly testing the behavior of the ES client, is not trivial, as the exact behavior of the sniff/sniff on start logic is an implementation detail of the client.

For those reasons, I will close this.