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