Try to fix race condition in watcher test

The listen watcher version of this test seems to hiccup from time to
time, perhaps this will help.

Author: jonleighton

test/unit/watcher_test.rb

@@ -125,12 +125,12 @@ def test_on_stale
     file = "#{@dir}/omg"
     touch file, Time.now - 2.seconds
 
-    watcher.add file
-    watcher.start
-
     stale = false
     watcher.on_stale { stale = true }
 
+    watcher.add file
+    watcher.start
+
     touch file, Time.now
 
     Timeout.timeout(1) { sleep 0.01 until stale }