Bug 1517083 - Enable running custom tests on extension-based experiments; r=rhelmer,froydnj

Building off the work from Bug 1451159, this creates a new ./testing/extensions directory and adds it to the list of directories to check for moz.build files in ./toolkit/toolkit.mozbuild.

This will enable developers to run custom tests on their extensions locally and on the Try server by following the steps in the ./testing/extensions/README.txt file.

The ./testing/extension/moz.build file is required by the build system, but it will be overwritten with the developer's own moz.build file.

Differential Revision: https://phabricator.services.mozilla.com/D17568

--HG--
extra : moz-landing-system : lando

testing/extensions/README.txt

@@ -0,0 +1,4 @@
+To run custom Mozilla tests on an extension (Bug 1517083):
+1. Drop an extension XPI into testing/profiles/common/extensions (Bug 145119); this extension will be installed with the testing profile.
+2. Drop a moz.build file in this directory that registers any relevant manifests for any tests to be run for the extension. This will overwrite the placeholder moz.build file in this directory.
+3. Drop those tests into this directory.

testing/extensions/moz.build

@@ -0,0 +1,5 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# This file will be overwritten when adding custom extension tests.

toolkit/toolkit.mozbuild

@@ -186,6 +186,7 @@ if CONFIG['ENABLE_TESTS']:
         '/testing/modules',
         '/testing/runtimes',
         '/testing/web-platform',
+        '/testing/extensions',
     ]
 
     # The file id utility requires breakpad libraries.