tests: Extract get_user_id helper for import tests.
tests: Extract get_user_id helper for import tests.

The new helper sets us up to drop short_name.

punchagan/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,showell/zulip,kou/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,hackerkid/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,showell/zulip,kou/zulip,hackerkid/zulip,kou/zulip,rht/zulip,brainwane/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,rht/zulip,showell/zulip,rht/zulip,punchagan/zulip,brainwane/zulip,zulip/zulip,showell/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip