Add test cases for CheckSignatureEncoding

Summary:
Mostly covering the SigHashType handling so far. More tests required, but it's better than nothing.

Depends on D1566

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1567