[script] Unit tests for script/standard functions

Summary: This is a partial backport of Core PR11116 : https://github.com/bitcoin/bitcoin/pull/11116/commits/d7afe2d1577eb3f6775301c81b94a191e0db99d9

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3116