Scoru,Test: checks the challenge window period boundaries

Context

While looking at the constants declaration to tackle #2980 (closed), I was a bit surprised that the challenge window period is non-inclusive. I extended a test to expose this behavior (it can be fine to leave as is but we need to be sure about the boundaries).

IT WAS WRONG:

The challenge window perido is inclusive, if you wait:

• n - 1 blocks : NOT GOOD
• n blocks: GOOD 😄

So in summary, the mr is an integration test testing these bounds. I also included a small refactoring about if ... then fail ... patterns.

Manually testing the MR

The CI should prevail (but do not trust the flakiness :p).

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR