[test] add a test for a transaction failing specifically for scriptsig-not-pushonly

Summary:
Other transactions in invalid_txs.py fail block inclusion because of non-pushonly scriptsigs, but fail mempool inclusion for more specific reasons.
Craft a transaction that fails only for the pushonly rule, based on the existing test in chronik_electrum_blockchain (note that b"a" == b"\x61" == CScript([OP_NOP])

Depends on D18927

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18928