Fix tests failing when enabling template-only-glimmer-components.

These tests were attempting to modify the
`ENV._TEMPLATE_ONLY_GLIMMER_COMPONENTS` value at runtime, but load
different code when it is and isn't set. So now that the test suite is setting
`template-only-glimmer-components` when the Ember code loads we can no
longer "fake" these tests.

However, the changes to the test runner mean that we still test this
both on and off so we are not loosing coverage.