Make sigencoding_tests significantly faster by not checking the full sigashtype/flags combinatorial

Summary:
This required to expose checking of the signature without a hash type.

Depends on D1579

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1580