Cleanup GN BUILD files, gni's and imports.

mojo/runner/test/BUILD.gn

-import("//third_party/mojo/src/mojo/public/mojo.gni")
-import("//third_party/mojo/src/mojo/public/mojo_application.gni")
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//mojo/public/mojo_application.gni")
 import("//third_party/mojo/src/mojo/public/tools/bindings/mojom.gni")
-import("//testing/test.gni")
 
 mojom("bindings") {
   sources = [
     "pingable.mojom",
   ]
 }
 
 mojo_native_application("pingable_app") {
   output_name = "pingable_app"
 
   testonly = true
 
   sources = [
     "pingable_app.cc",
   ]
 
   deps = [
     ":bindings",
     "//mojo/application/public/cpp",
     "//third_party/mojo/src/mojo/public/cpp/bindings:callback",
   ]
 }