Add grunt test task for QUnit tests.

props kadamwhite.
see #25096.


git-svn-id: https://develop.svn.wordpress.org/trunk@25178 602fd350-edb4-49c9-b593-d223f7449a82

package.json

@@ -13,6 +13,7 @@
     "grunt-contrib-clean": "~0.5.0",
     "grunt-contrib-copy": "~0.4.1",
     "grunt-contrib-cssmin": "~0.6.1",
+    "grunt-contrib-qunit": "~0.2.2",
     "grunt-contrib-uglify": "~0.2.2",
     "grunt-contrib-watch": "~0.5.1"
   }