Update weird way to check for MTP in functional tests

Summary: I noticed that some of our tests check for MTP in a strange way. This updates them to do so in a more straightforward fashion.

Test Plan:

./test/functional/test_runner.py abc-mempool-coherence-on-activations abc-replay-protection

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5233