Add rudimentary shelf delegate mus unit test.

ash/mus/BUILD.gn

Index: ash/mus/BUILD.gn
diff --git a/ash/mus/BUILD.gn b/ash/mus/BUILD.gn
index da25a6e595f84e998d6d028189020d732b7ba3a6..35734a6e78825367a5c13b42e84758393b00798d 100644
--- a/ash/mus/BUILD.gn
+++ b/ash/mus/BUILD.gn
@@ -129,3 +129,23 @@ repack("resources_200") {
     "//ash/resources:ash_test_resources_200_percent",
   ]
 }
+
+source_set("unittests") {
+  testonly = true
+
+  sources = [
+    "shelf_delegate_mus_unittest.cc",
+  ]
+
+  deps = [
+    "//base",
+    "//components/mus/public/interfaces",
+    "//mash/shelf/public/interfaces",
+    "//mojo/public/cpp/bindings",
+    "//services/shell/public/cpp:shell_test_support",
+  ]
+
+  data_deps = [
+    ":mus",
+  ]
+}