test_events: Test the email event when changing delivery_email.
test_events: Test the email event when changing delivery_email.

We send three events when changing delivery email of a user - one
for updating the delivery_email field of user, one for avatar url
change, and one for changing email field if email_address_visibility
is set to 'EMAIL_ADDRESS_VISIBILITY_EVERYONE'.

There is already a test for delivery_email and avatar_url event with
the visibility setting set to 'EMAIL_ADDRESS_VISIBILITY_ADMINS_ONLY',
but no test for verifying the email update event sent when email
address is public, so this commit adds a test for checking the schema
of event for updating email field.

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