test_email_change: User create_confirmation_link for url.
test_email_change: User create_confirmation_link for url.

This is a prep refactor, instead of creating Confirmation
object and using `confirmation_url` for generating confirmation
link/url, using `create_confirmation_link` would be a cleaner
approach, also this can help us avoid failing test in case
Confirmation model is changed.

Part of #16359.

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