tests: Fix compatibility with openapi_core 0.13.7

tests: Fix compatibility with openapi_core 0.13.7

It seems the 'openapi_core.schema.schemas.models.Format' mechanism of
defining custom formatters was deprecated in openapi_core 0.12.0 but we
never noticed. They've finally broken it in 0.13.7. Switch to the new
thing, 'openapi_core.unmarshalling.schemas.formatters.Formatter', which
expects a slightly different format.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #395
(cherry picked from commit d11ac34e9f0f2a86901ed74da27fa7a2df109728)