Adapted from BSD0 licensed
https://gist.github.com/bojanz/00187fd502b75001953d200c11dbbd97, we can
create test helpers for validating that OpenAPI are still met by HTTP
request/responses in our `net/http` tests.

This removes a couple of things from that code, as well as makes it more
resilient for a few cases I've hit, such as the `Servers` array being
matched correctly, and ensures that response headers are validated.

This uses the [federated-api-model]'s OpenAPI contract, the Petstore
example, and some hand-crafted examples.

This follows best practices for CI/CD from micropub-go.

We use an interface, `testingT` to allow for easier stubbing in unit
tests. As `testing.T`'s interface requires using non-pointer methods, we
need to use `state` as a pointer, so we can update i.e. `errored`.

[federated-api-model]: https://github.com/co-cddo/federated-api-model