fix some tests that rely on free transactions being submittable via RPC

Summary:
Testing of the free transaction mechanism revealed that some tests were accidentally
relying on free high priority transactions being submitted via RPC, which
would be forbidden if they were submitted over P2P protocol.

Also, some tests were unnecessarily invoking the relaypriority option (what this
does is count *all* transactions as 'high priority' and therefore they are only
subject to the free limiter). There remains only one test (abc-p2p-compactblocks.py)
relies on -norelaypriority, and this is not due to RPC submission but rather
because the way it works is a bit weird as noted (it makes giant transactions with
0 fee, and expects them to all be put back into mempool when a reorg happens).

Test Plan:

test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4744