Refactor signature encoding checks to separates checks of the SigHashType and other checks
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
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D1578