Add tests for Application#injectTestHelpers.

Adds tests to ensure that the `onInjectHelpers` callbacks are ran, and
ensures that the tests are run in a consistent state (prior to the
`cleanup` changes here we would leak `ajaxStart` and `ajaxStop` event
handlers).
Merge pull request #3576 from rjackson/add_tests_for_on_inject_helpers

Add tests for Application#injectTestHelpers.