Make a mojo:apptests GN meta-target, add apptests.

mojo/BUILD.gn

Index: mojo/BUILD.gn
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index 66e69aff2f456a6977905259a1e28fc36b8a1672..e3eff72636b04ea9e56d52114e5133bf72288ff2 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -39,3 +39,20 @@ group("tests") {
     "//third_party/mojo/src/mojo/edk/test:mojo_public_utility_unittests",
   ]
 }
+
+# TODO(GYP): Not included in the mojo group meta-target; missing gyp targets.
+#            See the gn_all and gn_only comments in //src/BUILD.gn.
+group("apptests") {
+  testonly = true
+  deps = []
+  if (!is_component_build) {
+    deps += [
+      "//mojo/services/clipboard:apptests",
+      "//mojo/services/html_viewer:apptests",
+      "//mojo/services/network:apptests",
+      "//mojo/services/view_manager:mojo_view_manager_client_apptests",
+      "//mojo/services/view_manager:view_manager_service_apptests",
+      "//mojo/services/window_manager:window_manager_apptests",
+    ]
+  }
+}