Implement tests for Event Registration pages

CRM/Event/Form/Registration/Register.php needs to be refactored like CRM/Contribute/Form/Contribution.php so that we can implement tests on submit.

CRM_Event_Form_Registration_Register::postProcess() needs splitting up to call a new function CRM_Event_Form_Registration_Register::submit().

Then we need to add CRM_Event_Form_Registration_Register::testSubmit() and write some tests to call this function.