CDRIVER-4353 unskip `/server_discovery_and_monitoring/monitoring/heartbeat/pooled/dns` (#1440)

* unskip `/server_discovery_and_monitoring/monitoring/heartbeat/pooled/dns`

* use `.invalid` TLD in test

To reliably return DNS error

* remove unnecessary test skips

Test expects to fail DNS lookup. Does not require internet.

* remove unused `test_framework_skip_if_rhel8_zseries`

* reduce serverSelectionTimeoutMS to speed up test

* update comment and expected error

* measure duration after destroy client or pool

Destroying the client or pool results in sending `endSessions`. This may result in another heartbeat attempt.