[backport#12437] [Trivial] Simplify if-else blocks and more descriptive variable naming

Summary:
97bcd36811 [Trivial] Simplify if-else blocks and more descriptive variable naming (Jeff Rade)

Pull request description:

Was looking through `test_runner.py` to start work on [#11964](https://github.com/bitcoin/bitcoin/issues/11964).  Made a few changes to make the file more readable and keep these separate from future PR.

Backport of Core PR12437

Test Plan:

ninja all
test_runner --extended|no parameters|list_of_tests|-x list_of_tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8213