Add inline docs to test interface type definitions

Fixes #1686.

Updates the assertion skipping type definitions to be in line with the actual implementation.

Allow `.end()` in `CbExecutionContext` to take an error value.

Fix hook typing so it's clearer plain implementations can be used without a title. This was possible before through a macro, which was misleading.

Co-authored-by: Arjay Sitoy <sitoy.arjay@gmail.com>
Co-authored-by: Mark Wubben <mark@novemberborn.net>

Author: kugtong33