fix a test in anticipation of SCRIPT_VERIFY_INPUT_SIGCHECKS activation

Summary:
Currently we have just one test which creates an abnormal 'abusive'
script of the kind that SCRIPT_VERIFY_INPUT_SIGCHECKS (see D4617) is
designed to block from the mempool.

This adds padding to the P2SH script thereby pushing the scriptSig
length up to at least 198. ( This script does 6 sigchecks, and
6 * 43 - 60 = 198 ; note the resulting density for this input,
including the other input fields, is 6 / (41+198) = 1/39.8
sigchecks/byte )

Test Plan:

ninja all
test_runner.py abc-mempool-accept-txn

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5016