Rename activation tests which are now feature tests

Summary:
Follow-up for D4642, see
https://reviews.bitcoinabc.org/D4642?id=14901#inline-28944.

Depends on D4642.

Test Plan:

./test/functional/test_runner.py abc-minimaldata abc-schnorrmultisig

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4747