Add unit test for mus_demo.

services/ui/demo/test_manifest.json

Index: services/ui/demo/test_manifest.json
diff --git a/services/ui/demo/test_manifest.json b/services/ui/demo/test_manifest.json
new file mode 100644
index 0000000000000000000000000000000000000000..7e4ca6523ba60d72f9f2508eb7eef51e66689eb1
--- /dev/null
+++ b/services/ui/demo/test_manifest.json
@@ -0,0 +1,11 @@
+{
+  "manifest_version":  1,
+  "name": "exe:mus_demo_unittests",
+  "display_name": "MUS Demo Unittests",
+  "capabilities": {
+    "required": {
+      "*": { "classes": [ "app", "test" ] },
+      "mojo:ui": { "interfaces": [ "ui::mojom::WindowServerTest" ] }
+    }
+  }
+}