failing test for contribute_to_field populating constraints

[olivierdalang]

For now this is only a failing test case to run against official CI. Hope it's OK to open such a PR. At some point this will hopefully become a PR to support having fields populating DB constraints in contribute_to_fields.

See https://forum.djangoproject.com/t/dynamically-populate-constraints-in-field-contribute-to-class/22934 for full context.

[olivierdalang]

(needs more work, will come back later)