Add rudimentary shelf delegate mus unit test.

mash/unittests_manifest.json

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