This adds a new /js-tests/extensions/ page that runs through the test
suites for all enabled extensions. It will load the "tests" bundle (if
it exists) from each extension, along with any default or included
bundles.

JSExtensions classes are not initialized on this page. Instead, test
suites should define them with whatever data is necessary to test
against, like any other model. It's up to the extension author to ensure
that any JSExtensions they test can be run within the context of the
test page.

Added some unit tests to an extension through a "tests" bundle and

visited the new page. Saw the tests run.