Disable wxSleep() test case for buildbot builds.

This test keeps failing just because the buildbot machine are too heavily
loaded. This is not really an error, so just avoid running this test there.

This is the backport of f641dfd7 from master.

tests/events/stopwatch.cpp

@@ -64,6 +64,14 @@ CPPUNIT_TEST_SUITE_NAMED_REGISTRATION( StopWatchTestCase, "StopWatchTestCase" );
 
 void StopWatchTestCase::Misc()
 {
+    // Buildbot machines are quite slow and sleep doesn't work reliably there,
+    // i.e. it can sleep for much longer than requested. This is not really an
+    // error, so just don't run this test there -- and if you get failures in
+    // this test when running it interactively, this might also be normal if
+    // the machine is under heavy load.
+    if ( IsAutomaticTest() )
+        return;
+
     wxStopWatch sw;
     long t;
     wxLongLong usec;