main.mdev375, main.connect fail on macOS

Fix main.mdev375, main.connect so they're reliable on macOS.

Update: Originally posted https://github.com/MariaDB/server/pull/5008 as a way to relax the test constraints on macOS as the problem surfaces only on that system. Through a conversation on that PR, we discovered a couple of other possible approaches. Ideally, we would fix the client library to use select or poll like most other systems that rely on networking.