update testing guide for REST clients

While looking at https://github.com/googleapis/google-cloud-go/issues/9188 I noticed that our testing guides for this repo do not include examples for our REST based clients. Something very similar to https://github.com/googleapis/google-api-go-client/blob/main/testing.md#testing-http-services-using-fakes should work, but we should update the guide to be more complete now that almost all of our clients support REST/HTTP.