sort python imports in feature_* functional tests

Summary:
This is preparation work to add a new linter that will enforce this new sorting.

The command used was (excluding contrib/):

isort --profile=black --line-length=79 test/functional/feature_*

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10201