test_signup: Refactor test_signup_without_full_name to use verify_signup.
test_signup: Refactor test_signup_without_full_name to use verify_signup.

The test now uses submit_reg_form_for_user, meaning a blank
full_name is posted to /accounts/register/ rather than the
parameter being excluded.

Fixes part of #7564

rht/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip