Improve tests for the string format linter

Summary:
Add high level doctests to check that detection is working as expected.

Depends on D2025

Test Plan:

python3 -m doctest -v test/lint/lint-format-strings.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2225