Ensure testing elements are properly reset/cleared.

[rwjblue]

Prior to this change, the innerHTML of the #ember-testing element was reset after each test (much in the same way as QUnit itself resets the #qunit-fixture between tests). There were two specific issues with the approach:

1. The teardownRenderingContext was required to run after the teardownContext. If the order was set correctly (teardownRenderContext(this) then teardownContext(this)) errors would be thrown because Ember is attempting to clean up the rendering engine and its expected DOM elements to remove are no longer present. This meant that we could not use the normal rule of thumb for these things: that teardown is done in the reverse order of setup.
2. Any attributes or properties added to the #ember-testing element were not being cleaned up, and caused a large number of cascading test failures. Ember's acceptance tests set an ember-application class on what it thinks is the rootElement, if it sees that class is already present it throws an error. Due to the way tests were being improperly cleaned up, one failed acceptance test (which failed in such a way as proper cleanup could not happen) would essentially cause all of the remaining acceptance tests to fail.

The fix here was to swap from capturing and resetting the #ember-testing elements innerHTML to capturing and resetting the #ember-testing-container's innerHTML. This has the effect of ensuring that the #ember-testing element (which is contained within the #ember-testing-container element) is reset completely, and allows Ember's rendering system to continue to do its normal cleanup without error (because the elements it expected to exist are still present).