[tests] fix flake8 warnings in interface_rest.py test

Summary:
While at it, also fix nits in the comments.

Partial backport of core PR12766 (commit abf190e)
https://github.com/bitcoin/bitcoin/pull/12766/commits/abf190e4e7d79be6f4749dec24a3933e7a8a4507

Depends on D3764

Test Plan:

./test/functional/test_runner.py interface_rest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3765