Enable Schnorr signature verification in CHECK(DATA)SIG(VERIFY)

Summary:
previously in D2469 the sigencoding changes were done; this completes the
schnorr implementation by modifying VerifySignature. Mostly this is a battery
of tests on the opcodes themselves.

Test Plan:
added numerous tests in script_tests.
also, new opcode-probing module that uses pseudorandom flags testing.

Reviewers: #bitcoin_abc, deadalnix, Fabien, Mengerian

Reviewed By: #bitcoin_abc, deadalnix, Fabien, Mengerian

Subscribers: Fabien, Mengerian, jasonbcox, teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2455