add SCRIPT_ENABLE_SCHNORR_MULTISIG flag for new multisig mode

Summary:
Additionally, a variety of tests are included for ensuring legacy
multisig behaviour continues unaltered under the new flag (when dummy=null).

Test Plan: make check

Reviewers: deadalnix, Mengerian, jasonbcox, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Maniphest Tasks: T528

Differential Revision: https://reviews.bitcoinabc.org/D3472