add test script for multisig delegator to single sig delegatee

[lenkan]

~~The dip event never ended up in the delegables escrow. See change in eventing.py.~~

Same fix was merged in #997.

This PR now only adds a test script for multisig delegator to single sig delegatee using separate witnesses for delegator and delegatee.

See also #992, I use the same utility functions and approach to test script here. I can change the other scripts to be implemented this way as well. I can also just put these scripts in a separate repository. Let me know.