Many of the client tests that use the mock server currently fail and/or are flaky. The problematic tests are:

/Client/fetch_stream/retry/fail
/Client/command/read_prefs/pooled
/Client/mongos_seeds_reconnect/single
/Client/mongos_seeds_connect/pooled
/Client/exhaust_cursor/err/server/2nd_batch/pooled
/Client/rs_seeds_connect/pooled

Some of the error messages that are produced include:

request_matches_msg(): precondition failed: request

hang up! monitor: [127.0.0.1:59723] command or network error occurred: Failed to read 4 bytes: socket error or timeout

All of the tests terminate in a way that makes it seem like the mock server is hanging up prematurely. These failures could very likely be related to CDRIVER-4111.

They are currently being skipped, so to work on them, it will first be necessary to remove them from .evergreen/skip-tests.txt and run a patch build to determine the current failure mode(s).