tests: add functional test for miniscript decaying multisig
This is very closely based on 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).