Relax wxStopWatch test to make it less likely to fail on buildbot.

Be prepared for sleeping taking more time than we budget for it on a highly
loaded machine such as a buildbot slave.

tests/events/stopwatch.cpp

@@ -111,7 +111,7 @@ void StopWatchTestCase::Misc()
     WX_ASSERT_MESSAGE
     (
         ("Actual time value is %ld", t),
-        t > 2*sleepTime - tolerance && t < 2*sleepTime + tolerance
+        t > 2*sleepTime - tolerance && t < 3*sleepTime
     );
 }