test: Move P2SH_OP_TRUE to shared test library

Summary:
Otherwise it can't be used in other tests (unit, fuzz, bench, ...)

Backport of core#21211.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18312