Flaky socket timeout tests

[shamblett]

The socket timeout tests introduced on #589 run OK locally but fail on GitHUb first time around, pass OK if re run.

✅ test/mqtt_client_connection_unsecure_test.dart: Client interface Socket Timeout

Error: 291 tests passed, 1 failed.
Unhandled exception:
ProcessException: 
  Command: dart run --pause-isolates-on-exit --disable-service-auth-codes --enable-vm-service=8181 test
#0      _dartRun (file:///home/runner/.pub-cache/hosted/pub.dev/coverage-1.11.1/bin/test_with_coverage.dart:35:5)
<asynchronous suspension>
#1      main (file:///home/runner/.pub-cache/hosted/pub.dev/coverage-1.11.1/bin/test_with_coverage.dart:217:3)
<asynchronous suspension>
Error: Process completed with exit code 255.