Provide capability classes from Mus

mash/unittests_manifest.json

 {
+  "manifest_version":  1,
   "name": "exe:mash_unittests",
   "display_name": "Mash Unittests",
-  "capabilities": { "*": ["*"] }
+  "capabilities": {
+    "required": {
+      "mojo:desktop_wm": { "interfaces": [ "*" ] },
+      "mojo:quick_launch": { "interfaces": [ "*" ] },
+      "mojo:mus": {
+        "classes": [ "mus:core" ],
+        "interfaces": [ "mus::mojom::WindowServerTest" ]
+      }
+    }
+  }
 }