Refactor script_tests.cpp to separate signature generation from PushSig
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

Committer's notice: Adapted to BU.

Co-authored-by: Awemany <d04faf01443ef4632432d4c4aa734f873cc405fb@protonmail.com>

BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited