tests: Provide a way to disable API schema

tests: Provide a way to disable API schema

The API schema validation is strict, in that it will error out with
invalid keys in either the request or response. Unfortunately the API
itself is not. We're hopefully going to fix this in a distant v2.0, but
for now we need a way to ensure that the API does what it's supposed to,
namely not set fields that don't exist or that the user isn't allowed to
set, even if proper error codes aren't raised.

This isn't actually used yet. That will come later.

Signed-off-by: Stephen Finucane <stephen@that.guru>