[schnorr] Add verification routine to the test framework schnorr signature facility

Summary: This ensure we also have facility to check the validity of a schnorr signature.

Test Plan: By modifying the signature routine, we ensure that this code self check often when runnign the test suite.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5536