DEV: Destroy application instance after each test (PR #14455)

Under Ember CLI, we create a new application instance for each test. We were not correctly destroying it after the test, causing many references to be maintaned (e.g. at the end of a test run, Ember.Namespace.NAMESPACES would have an entry for each application instance).

Calling destroy on the application instance tidies up these references, and is one step towards fixing our test memory leak problem. Unfortunately there still seem to be other references being held to the application, so this commit is not a total fix.


Note that we have something similar in our other ember apps:

After some talks with @davidtaylorhq, LGTML, also it’s only in tests so not very risky.