[lint] Fix variables shadowing python builtins in the test framework

Summary: Shadowing builtins can lead to unexpected behavior. Continuing work from D13740

Test Plan:

ninja check-functional
flake8 test_framework/ | grep A001  # empty

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13743