[build-bot] Use unittest.assert* instead of plain asserts

Summary:
Using the unittest module's assert methods provides more informative
and/or easier to understand error messages. We already use them in most places,
so this diff cleans up the remaining ones.

There is no change in behavior.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8028