[script_tests] improve test coverage of minimal push rules

Summary:
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