This is similar in structure to test/functional/wallet_multisig_descriptor_psbt.py
both in code and concept. It should serve as some integration testing for
Miniscript descriptors, and also documents a simple multisig that starts as 4-of-4
and decays to 3-of-4, 2-of-4, and finally 1-of-4 at block heights (I think in the
real world aligning this to halvenings would be nice).
