add a couple more Schnorr checkmultisig tests

Summary:

• 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