add SCRIPT_ENABLE_SCHNORR flag

Summary:
This merely adds the flag -- no change in behaviour yet.
Numerous tests are duplicated in script_tests to make sure that nothing
weird happens when the flag is merely activated.

Test Plan: run test_bitcoin

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, Fabien, teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2430