Convert tests to use schema functions

[adriangb]

A lot of our tests use plain dicts. This makes them brittle and hard to refactor. We should convert from {'type': 'str'} -> core_schema.str_schema().

Selected Assignee: @davidhewitt

[sydney-runkle]

This is going to be incredibly laborious 😭

[davidhewitt]

Done in #1631, thanks @mikeedjones!