test_signup: Attach prereg_user to confirmation obj.
test_signup: Attach prereg_user to confirmation obj.

Tests attached a UserProfile to confirmation objects,
which is not very valid as this is the only place
where this is done. Now we attach PreregUser to
the confirmation object, making the tests correct.

Co-authored-by: Arunika <e37b6aba96cf4a7f6831350072876dc7359682d5@gmail.com>

hackerkid/zulip,zulip/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,andersk/zulip,synicalsyntax/zulip,showell/zulip,andersk/zulip,kou/zulip,punchagan/zulip,kou/zulip,rht/zulip,showell/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,eeshangarg/zulip,rht/zulip,showell/zulip,andersk/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,showell/zulip,andersk/zulip,brainwane/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,kou/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,shubhamdhama/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,synicalsyntax/zulip,synicalsyntax/zulip,brainwane/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,timabbott/zulip,rht/zulip,zulip/zulip,timabbott/zulip,shubhamdhama/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip