test: Adapt test framework for chains other than "regtest"

Summary:

• test: Fix “local variable 'e' is assigned to but never used”

flake8 F841 lints, as of flake8 3.6.0

• test: Adapt test framework for chains other than "regtest"

Co-Authored-By: Jorge Timón <jtimon@jtimon.cc>

• test: Avoid hardcoding the chain name in combine_logs

This is a backport of Core PR16509

The original PR contains a lot of formatting that was made obsolete by our process.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5942