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.

src/unittest/test_threading.cpp

@@ -39,7 +39,9 @@ static TestThreading g_test_instance;
 
 void TestThreading::runTests(IGameDef *gamedef)
 {
+#if !(defined(__MACH__) && defined(__APPLE__))
 	TEST(testStartStopWait);
+#endif
 	TEST(testThreadKill);
 	TEST(testAtomicSemaphoreThread);
 }