subdomains: Write some tests for get_subdomain.
subdomains: Write some tests for get_subdomain.

This logic is a bit subtle, and we're about to make changes to it.
So let's have some tests.

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