[tests] Add uacomment tests

Summary:
Checks for setting the value, max length and reserved characters.
The backported test has been modified to match our method, not throwing
exception but truncating and sanitizing the comment.

Backport of core PR11486

Depends on D2275

Test Plan:

./test/functional/test_runner.py uacomment

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2276