Add test case for CheckSignatureEncoding for proper DER encoding

Summary:
This ensures IsValidSignatureEncoding is properly checked.

The test time is atrocious because IsValidSignatureEncoding assumes the presence of the sighash type which causes a combinotary explosion, and also uncover a SRP violation. I would like to refactor these to detangle the two, but obviously doing so safely require a test so here we are.

Depends on D1570

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1571