[BUGFIX] Fix an issue with concatenatedProperties.

This commit fixes an issue with how contactedProperties are applied that
can result in subclasses sharing a reference to the same array, in a
surprising way. As an example, the failing test provided here shows
how two Ember.App subclasses shares were sharing inadvertently
sharing initializers.

Thanks to @kselden and @stefanpenner for code review.
Add assertion upon too many ajaxStop's.

This is should have helped in #3564.

Conflicts:
	packages/ember-testing/tests/helpers_test.js
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).

Conflicts:
	packages/ember-testing/tests/helpers_test.js