add SCHNORR_MULTISIG to mandatory flags

Summary:
Note that this makes pre-activation mempool behaviour slightly weird, as
noted in the test. In practice now that the upgrade has passed and is
checkpointed, only a new node during IBD can be in a preactivated state,
and a new node will anyway not request transactions before finishing IBD.

It may be worth adding tests of fresh-node IBD behaviour in a future
commit; similar behaviour occurs for any additive flag that is included
in mandatory flags.

Test Plan:

make check
test_runner.py

Reviewers: Mengerian, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D4673