[BUGFIX beta] Use injected test helpers instead of local functions.

Since we are injecting the application with our helpers (into the
`App.testHelpers` hash), we should be using that helper and not simply
calling the function in local scope.

As the tests show, this allows a user to override the internal helpers
with a taylored version for their scenarios.
Merge pull request #4520 from rjackson/allow-custom-helpers-to-override-defaults

[BUGFIX beta] Use injected test helpers instead of local functions.