add a couple more Schnorr checkmultisig tests
- a test with a one-byte checkbits not pushed using OP_1-OP_1NEGATE.
- a test with a two-byte checkbits.
Also re-disable auto script gen (was inadvertently activated in D3474).
The auto script gen should generally be kept off since it allows the tester
to emit "Missing auto script_valid test" in case that the generator has
been changed somehow and impacted existing tests.
Test Plan: make check
Reviewers: deadalnix, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4455