Bug 886168 - Turn on webaudio tests on B2G. r=ehsan

We must do this by blocking the other subdirs with tests in
content/media.

testing/mochitest/b2g.json

@@ -9,7 +9,8 @@
 "excludetests": {
     "content/xbl/":"",
     "content/xul":"",
-    "content/media/":"",
+    "content/media/test":"",
+    "content/media/webspeech":"",
 
     "dom/imptests/editing/selecttest/test_addRange.html": "oom?, bug 775227",
     "dom/imptests/webapps/DOMCore/tests/approved/test_Range-insertNode.html":"oom?, bug 775227",