Add test for CheckSignatureEncoding to verify low S is checked properly.

Summary:
Increase the unit test coverage for CheckSignatureEncoding.

Depends on D1567

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1570