[NFC] Add test for new case email url

[demeritcowboy]

Overview

Just a basic form open test

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[demeritcowboy]

CRM_Case_Form_EmailTest::testOpeningEmailForm
Exception: CRM_Core_Exception: "Case does not have a recognized case-type!"

Right.... that same error was why the other day I gave up and moved a test into BAO_CaseTest to avoid dealing with it.
Hmm.