Update feature_* functional tests to use the new .format() method

Summary: Avoid using % to format strings, switch to the new method instead.

Test Plan:

./test/functional/test_runner.py feature_assumevalid feature_cltv

feature_dbcrash feature_dersig feature_minchainwork
feature_notifications feature_proxy feature_pruning

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2568