Need to inject config:environment into all unit tests that touch Apollo

[bgentry]

Follow-up from #53.

It's a bit annoying to have to use a needs: ['config:environment'] in addition to already needing a service:apollo in any unit test that touches Apollo. As of now, this blows up on the config.apollo call when not using that injection because config is undefined here.

One way I'm working around this in my own tests is with this hack:

// used in environments without injected `config:environment` (i.e. unit tests):
const defaultOptions = {
  apiURL: "http://testserver.example/v1/graph",
};

export default ApolloService.extend({
  options: computed(function() {
    let config = getOwner(this).resolveRegistration("config:environment");
    if (config && config.apollo) {
      return config.apollo;
    } else if (testing) {
      return defaultOptions;
    }
    throw new Error("no Apollo service options defined");
  }),
  // ...
});

I'm not sure if there's a better way to handle this. At the very least, I think there should be a descriptive error like this when a required config isn't set up or available.

[dfreeman]

One alternative would be to instruct folks to use integration: true rather than needs: [...] in their apollo-related unit tests, but that may feel kind of weird until it becomes the documented norm and integration goes away.

Either way, falling back to default options in tests seems like an ergonomic win 👍