tests: don't repeat scriptPubKey value

Summary:
Use a variable with a meaningful name to improve readability.

See https://reviews.bitcoinabc.org/D8929#204886

This is a partial backport of Core PR11423
364bae5f7a6b16eef63990154e48f19e7e693039

Test Plan: test/functional/test_runner.py p2p_invalid_tx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8934