Use `Application.buildInstance` if possible to create `owner` for use in tests.

[rwjblue]

Application.prototype.buildInstance is exactly what is done in "reality" to build the owner object. We should migrate to a setup where the host application sets up the application under test (essentially replacing what is done viasetResolver) in its test-helper.js and then for each test we do something like this.owner = previouslyCreatedApplicationInstance.buildInstance();.

This would essentially provide the following benefits:

• Run initializers removing the (fairly large) divide between "acceptance" and "integration" tests
  • Automatically run initializers once (for the entire test suite)
  • Run instance-initializers per-test
• Share the registry for all tests (avoiding wasted work per test)
• Align our setup process for all test types in preparation for handling acceptance tests in the same manner.
• Remove the need for significant amounts of private API usage around Ember._RegistryProxyMixin / Ember._ContainerProxyMixin