add tests to ensure that API specs are OpenAPI 3.0 compliant

[bhearsum]

We've recently started using OpenAPI-style specs to define both our admin and public APIs. We use Connexion to load and create routes for them in Flask. It turns out that Connexion has some extensions that make it possible for us to have Connexion-compatible specs that aren't OpenAPI 2.0 compliant. The consequence of this is that we cannot make use of other OpenAPI 2.0 tools, such as https://github.com/swagger-api/swagger-codegen.

We're still working on becoming OpenAPI 2.0 compliant, but we should add tests to make sure it doesn't regress once we get there.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1387063)

Need to wait for #1084 first.