test: use current user in the multi-user amend scenario
The obsfate template keyword that will be introduced in a later patch
distinguish between current user and other users when displaying the obsfate
users.
Update the test before for clarity.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}