Fix channelgetallusermentionsjobtest by comparing strings rather than URLs

Summary:
gdb says one has

query = "roomId=avat&count=5&offset=12&sort={\"foo\":-1}",

and the other one has

query = "roomId=avat&count=5&offset=12&sort=%7B%22foo%22:-1%7D",

Using decoded characters like '{' in the literal URL doesn't help, they
get encoded anyway. I guess one would have to use fromEncoded or special
flags to prevent that from happening. Oh well, the string representation
is what we really care about in the end (I suppose).

Test Plan: ctest is green again

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: Ruqola

Differential Revision: https://phabricator.kde.org/D21359