Improve JSON testing for NamedOperationResolver

[GCHQDeveloper314]

Describe the improvement you'd like
Testing for NamedOperationResolver should cover configuration of a named operation depth using JSON.

Why do you want this improvement?
Although not yet documented, if admins want to adjust the depth this is currently the only way to do it. As such it should be fully tested.

Additional context
See #3174 where the depth limit was added and #3176 which might indicate part of how to add this testing.