[script_tests] improve test coverage of minimal push rules
It looks like the MINIMALDATA flag is destined to be a consensus rule so
it needs to be tested properly.
Some of the features of MINIMALDATA push restrictions were not being tested fully, namely there apparently were only tests for scriptSig.
Test Plan: make check
Reviewers: deadalnix, jasonbcox, Fabien, Mengerian, #bitcoin_abc
Reviewed By: jasonbcox, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D3350