[backport#14356] fix converttopsbt permitsigdata arg, add basic test

Summary:
The final check for extraneous sigdata has a flipped boolean, resulting in incorrect behavior.

This is a backport of Core PR14356

Test Plan:

ninja check
./test_runner.py rpc_psbt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6026