Why should I use a separate test host for running XCTests and how should I do that?

Question

I once asked a question related to XCTests. And in one of the answers I was told that it is a common practice to use a separate test host (other than the main app) when runn

Answer 1

Reasons to use a different host app for unit tests:

1. There is no main app, because your target is a library.
2. When the main app is launched, it goes through its start-up process. This takes too long, and has side-effects.

Reason 1 is pretty much a given. But what about reason 2?

My approach is to use the main app, but use a different application delegate during testing. That way I don't have to maintain a separate app, just an alternate start-up process. See How to Easily Switch Your App Delegate for Testing