Factorize the tx use count checks in blockencodings_tests

Summary: This is a simple code deduplication, that will make the follow-up diffs easier to review.

Test Plan:

ninja check-bitcoin-blockencodings_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13436