(fix) tests: fix flaky test_no_connection_refused_on_timeout

The test asserted that at least one WriteTimeout/WriteFailure must occur
during 2000 concurrent LWT operations, but this is non-deterministic --
if the cluster handles all ops without timeout, the assertion fails.

The actual purpose (PYTHON-91) is to verify the connection remains
functional after LWT timeout stress. Replace the timeout assertion with
a post-execution liveness check that verifies the session can still
execute queries, and log a warning when the timeout path is not
exercised.

Author: mykaul