Afform - Fix custom field handling and add tests

[colemanw]

Overview

This ensures custom fields are handled properly by Afform, including multi-record custom field groups & their autogenerated blocks, and contact reference fields.

Before

Custom contact reference fields not working in Afform.

After

Fixed

Comments

The bulk of this commit is unit tests :)

[civibot]

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[eileenmcnaughton]

OK this looked pretty munted without this patch

& worked with it - i created a form & created a contact with a contact reference field & it worked