test docs: clarify ngOnChange hook under test

[mmmichl]

Coming from the issue angular/angular#9866 there should be a clarification in the docs as how the ngOnChange hook is handled during a component test.

For a lot of people (including me) it is not obvious, that ngOnChange is not called under test, therefore please provide a clarification in the docs. It would be great to provide some strategy how to test a component which depends on the ngOnChange hook.

Keep up the good work 😄