Work around failing file watcher test under XP.

Under only Windows XP the test FileSystemWatcherTestCase::TestTrees fails
during the RmDir(treedir) call (SHFileOperation strangely returns
ERROR_DIR_NOT_EMPTY). To make the test pass remove the treedir first and
only then the singledir. This is merely a workaround while the actual
problem is still to be investigated.

tests/fswatcher/fswatchertest.cpp

@@ -922,8 +922,8 @@ void FileSystemWatcherTestCase::TestTrees()
             RemoveAllWatches();
 
             // Clean up
-            RmDir(singledir);
             RmDir(treedir);
+            RmDir(singledir);
 
             Exit();
         }