Between Feb 12th and Feb 18th 2020 tests on macOS started failing. I.e. between these two commits:
https://evergreen.mongodb.com/version/mongo_c_driver_6cd21d27864ae7058f9db8d1ba1132ee440fe804
https://evergreen.mongodb.com/version/mongo_c_driver_e69fd8960da6c11ef2ac22b662218da4e7cd3692

Tests that are time sensitive frequently fail:

• /Thread/cond_wait
• /Topology/try_once/succeed
• /Topology/connect_timeout/succeed
• /Sleep/basic
• /server_discovery_and_monitoring/monitoring/heartbeat/pooled/failed

I believe this is due to an environment change in Evergreen, but I have yet to determine what environment change is relevant, and whether there is a better workaround aside from increasing the timeouts/skipping the tests.

Observations

• The test /Thread/cond_wait is a very isolated test. That simply checks that calling mongoc_cond_timedwait with a 100ms timeout returns within 150ms. It now almost always fails due to returning after 150ms.
• Similar failures were observed in CDRIVER-3520 on Travis.
• Though the overall task runtime seems to have increased significantly (from ~4min to ~20min), most of the time in the failure is spent stuck in uploading, not running tests. The test runtime does not seem to be much longer.