test_signup: Refactor test_signup_existing_email to use verify_signup.
test_signup: Refactor test_signup_existing_email to use verify_signup.

Add a new `verify_signup` helper function, which currently implements
enough functionality to be used by `test_signup_existing_email`.

This is the first step towards #7564.

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