fix txvalidationcache_tests to not rely on NULLDUMMY

Summary:
DISCOURAGE_UPGRADABLE_NOPS is a much more reliable flag for testing
standardness in long term

Test Plan: make check

Reviewers: jasonbcox, #bitcoin_abc, deadalnix

Reviewed By: jasonbcox, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4451