Temporarily disable "testStartStopWait" Threading unit test on mac

The "testStartStopWait" unit test is unreliably failing on mac,
for some time already. See bug #3786.

Having the unittest fail unreliably doesn't help anybody but mostly
inhibits the main feature of travis builds: to test PRs for regressions.

Therefore, disable the specific unit test for until bug #3786
is fixed.