mash: Add a launch-test for ash_sysui.

mash/unittests_manifest.json

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