fix ineffective checkdatasig_tests

Summary:
These tests were totally broken:

• Only did 1 iteration because it set flags |= ENABLE_CHECKDATASIG at start
• Wasn't aware of SCRIPT_VERIFY_COMPRESSED_PUBKEYTYPE
• Missed some SCRIPT_VERIFY_NULLFAIL cases

Also I changed it to use pseudorandom flags generator since the original intent of 262000 iterations would have been way way too slow anyway. Runtime is now ~3 sec, would have been 3 minutes (dominating test_bitcoin runtime) if original iter count ever actually worked.

Test Plan: test_bitcoin

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2482