Speedup dbclient_rs_test

dbclient_rs_test is extremely slow. The reason is that it runs several find requests and wait 15 seconds for each of them to fail with FailedToSatisfyReadPreference.

The 15 seconds timeout comes from the ReplicaSetMonitorInterface::kDefaultFindHostTimeout.

Unittests are supposed to be quick because we run them as part of the commit tasks, also such a long test make unpractical to run unittest locally before to send an EVG patch.

A possible solution would be to use a failpoint to override the kDefaultFindHostTimeout in this specific unittest.