Use new expectDeprecation helper in tests, remove TESTING_DEPRECATION

Referencing https://github.com/emberjs/ember-dev/pull/121

This uses `expectAssertion` instead of a manual check in one location, drops a swp file, and introduces use of `expectDeprecation`.

`TESTING_DEPRECATION` was being used as a crutch (as far as my git digging can tell) to manage bad use of the raise-on-deprecate ENV setting. With the helper, things are mightily cleaned up and I think we can remove the flag.