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