Fix merging of global unknown data in PSBTs

Summary:
Actually merge the global unknown key-value pairs.

Add a test for merging unknown key-value pairs.

This is a partial backport of Core PR13721 : https://github.com/bitcoin/bitcoin/pull/13721/commits/fad231ad41e12b7047deb64220942ca8cb8357bc

Depends on D4456

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4457