tests: Add test for user signup with already-used email.
tests: Add test for user signup with already-used email.

[Test simplified a bit by greg.]

punchagan/zulip,andersk/zulip,mahim97/zulip,timabbott/zulip,punchagan/zulip,rishig/zulip,rht/zulip,dhcrzf/zulip,showell/zulip,synicalsyntax/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,andersk/zulip,rishig/zulip,tommyip/zulip,hackerkid/zulip,brainwane/zulip,tommyip/zulip,andersk/zulip,timabbott/zulip,rht/zulip,shubhamdhama/zulip,jackrzhang/zulip,tommyip/zulip,dhcrzf/zulip,synicalsyntax/zulip,jackrzhang/zulip,zulip/zulip,kou/zulip,mahim97/zulip,tommyip/zulip,synicalsyntax/zulip,rishig/zulip,jackrzhang/zulip,zulip/zulip,mahim97/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,showell/zulip,tommyip/zulip,rishig/zulip,zulip/zulip,showell/zulip,tommyip/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,dhcrzf/zulip,dhcrzf/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,rht/zulip,andersk/zulip,brainwane/zulip,tommyip/zulip,eeshangarg/zulip,hackerkid/zulip,timabbott/zulip,dhcrzf/zulip,rishig/zulip,brainwane/zulip,hackerkid/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,shubhamdhama/zulip,hackerkid/zulip,eeshangarg/zulip,mahim97/zulip,kou/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,jackrzhang/zulip,eeshangarg/zulip,rishig/zulip,mahim97/zulip,rht/zulip,kou/zulip,showell/zulip,dhcrzf/zulip,jackrzhang/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,shubhamdhama/zulip,jackrzhang/zulip,andersk/zulip,hackerkid/zulip,synicalsyntax/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,synicalsyntax/zulip,jackrzhang/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,mahim97/zulip