Yet another wxStopWatch unit test fix for buildbot.

Relax the check in RestartBug() test too, the time can be slightly smaller
than expected (by ~2ms on disc-xp machine).

This is the backport of 7af8598903 from master.

tests/events/stopwatch.cpp

@@ -145,7 +145,7 @@ void StopWatchTestCase::RestartBug()
     WX_ASSERT_MESSAGE
     (
         ("Actual time value is %ld", t),
-        t >= offset + sleepTime
+        t >= offset + sleepTime - tolerance
     );
 
     // As above, this is not actually due to the fact of the test being