Fix CRM/Event/BAO/AdditionalPaymentTest.php to use Order.create

Fix CRM/Event/BAO/AdditionalPaymentTest.php to use Order.create

This test wasn't really testing much since the functions were kinda contorted into doing what the
test was testing. I've updated it to do set up using Order.create & not to use the 'partial_amount_to_pay'
keys which turn out not to create valid transactions (& which we want to fully remove from the other places).

I had to co-erce tax_amount into a float in a couple of places & shortened a param for readability.

I commented out a couple of checks that only worked because of unrealisitic function calls
I added https://lab.civicrm.org/dev/financial/issues/102 to track that.