refactor(test): Use a single function to create email addresses.

* TestHelpers->createEmail will create a restmail address
* Update ad-hoc email address creations to use this
* Add TestHelpers->emailToUser to get the user from the email address.

fixes #1050
Merge pull request #1051 from mozilla/issue-1050-generic-email-create

refactor(test): Use a single function to create email addresses.