add sigcache tests

Summary: This adds tests for src/script/sigcache.* .

Test Plan:

• Make sure that cache hits when parameters are all repeated.
• Make sure that varying any of three parameters results in a cache miss.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2409