Refactor signature encoding checks to separates checks of the SigHashType and other checks

Summary:
This is done via slicing the sig to remove the SigHashType. Hopefully, the next step is to expose an API to check signature without the SigHashType which will allow to reduce the test time dramatically.

Depends on D1577

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1578