Running the tests for gr-change-actions would log hundreds of plugin
load timeout errors to the console. Because the errors would follow the
test completion and were asynchronously fired as part of initializing
the gr-change-actions element, they did not cause the tests to fail.
Stub the `awaitPluginsLoaded` method that was throwing the errors.
Change-Id: I980099ead6272e58269d62975122f8704bdc3e45