simplify checkdatasig_tests

Summary:
There's no need to force the sigops flag to be on during these tests, since
it has no impact on script evaluation.

Also, we really don't care whether or not the flag is in
MANDATORY_SCRIPT_VERIFY_FLAGS, because the code deciding ban/notban for inputs
that fail standard script evaluation does not care about sigops at all.

Test Plan: ninja check

Reviewers: Mengerian, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4908