Move SetMTP into test utils

Summary:
SetMTP is useful for testing any function that takes a block index and does its own activation checks.

It is possible to set MTP in unit tests with a naive implementation like:

for (auto &block : blocks) {
    block.nTime = target_mtp;
}

This works but has the possibly unintended effect of not testing activation boundaries reliably. We should encourage the use of SetMTP over the naive implementation. The first step to doing so is moving it into test utils.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13077