Refactor script_tests.cpp to separate signature generation from PushSig

Summary: It'll allow for addition of test for OP_CHECKDATASIG .

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1619