[BUGFIX] Allow keyboard events to work with the action helper.

The way action helper was validating the modifier keys against the
helper's allowedKeys option assumed a mouse/touch event. This commit
changes that to treat mouse/touch events differently.

Fixes #3340
Merge pull request #3663 from lukemelia/keyboard-actions

[BUGFIX] Allow keyboard events to work with the action helper
[FEATURE ember-testing-remove-helpers]

This adds a callback queue that fires upon removeTestHelpers. The main
reason for this change was to allow any cleanup from the
`onInjectHelpers` callbacks to be done.

For example removing the handlers for `ajaxStart` and `ajaxStop`.