Fix counter notify

[rasmus-kirk]

Purpose

It seems that the counter-notify example simply made the wrong check in
the test, note the following comment:

This will be false if the reentrancy has occurred

I added an enum to make the code clearer and fixed the assertion.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.