Fixed a flaky test: ZooKeeperTest.LeaderDetectorTimeoutHandling

- The original ZK session timeout was the same as AWAIT_READY timeout so it's possible that AWAIT_READY timed out in a race.
- Split the the test into two because to test detector we don't really need to wait several seconds for ZooKeeperTestServer to expire the session, which slows down the test.

Review: https://reviews.apache.org/r/24123