Add unit test for payment email from AdditionalPayment form, rationalise some paramet...

Add unit test for payment email from AdditionalPayment form, rationalise some parameters to do this.

Note that the goal is to move this function off this class to somewhere api accessible so breaking the connections
with the form makes sense & in fact some like the fromEmails handling are really just badly copied & pasted from elsewhere
(ie. the fromEmailIds doesn't exist & is created on the fly to house the email being passed in