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

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