Add a test of a behavior with fields to @inherit

[stevepiercy]

@erral @stevepiercy I considered the documentation tests to be a fairly adequate functional test. They confirm that the endpoint exists, can be called without an error, and returns data in the expected format -- and they'll break if that format changes unexpectedly.

As this issue revealed, they didn't cover the case of a behavior that actually includes fields. That was me being lazy because there wasn't an obvious built-in behavior that would be a good test case. It would be good to add a test that does this with a new behavior created for the purposes of the test. I'm drowning in tasks and probably won't get to this anytime soon.

Originally posted by @davisagli in #1895