tests: Replace messy direct test of login_or_register_remote_user.
tests: Replace messy direct test of login_or_register_remote_user.

This code path is much more naturally tested with the existing
end-to-end test for the function that we have for the RemoteUser auth
backend.

brainwane/zulip,hackerkid/zulip,jackrzhang/zulip,kou/zulip,synicalsyntax/zulip,showell/zulip,jackrzhang/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,zulip/zulip,andersk/zulip,rht/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,dhcrzf/zulip,timabbott/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,brainwane/zulip,shubhamdhama/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,hackerkid/zulip,dhcrzf/zulip,rishig/zulip,rht/zulip,rht/zulip,kou/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,andersk/zulip,showell/zulip,jackrzhang/zulip,dhcrzf/zulip,synicalsyntax/zulip,brainwane/zulip,jackrzhang/zulip,eeshangarg/zulip,punchagan/zulip,timabbott/zulip,eeshangarg/zulip,showell/zulip,zulip/zulip,showell/zulip,timabbott/zulip,tommyip/zulip,kou/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,jackrzhang/zulip,timabbott/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,rishig/zulip,kou/zulip,synicalsyntax/zulip,eeshangarg/zulip,dhcrzf/zulip,andersk/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,rht/zulip,shubhamdhama/zulip,tommyip/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,dhcrzf/zulip,jackrzhang/zulip,jackrzhang/zulip,synicalsyntax/zulip,rishig/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,tommyip/zulip,showell/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,dhcrzf/zulip,brainwane/zulip,synicalsyntax/zulip,shubhamdhama/zulip,brainwane/zulip,hackerkid/zulip,tommyip/zulip,brainwane/zulip,punchagan/zulip