Throw helpful error if Probot({id, cert}) are not set but `.app` is used, and throw error if `.app` is overwritten

Feature Request

Is your feature request related to a problem? Please describe.

This is a follow up for https://github.com/probot/probot/issues/883#issuecomment-473708953

For Probot 7 and below, we would override the .app() function to return a valid token. Now the .app() no longer exists. These tests now throw an error

TypeError: this.app.getInstallationAccessToken is not a function

Instead we now have to pass an app id numbers as id option and a valid private key as cert option.

Describe the solution you’d like

If the id and cert options are not passed, set a getter for probot.app and throw an error explaining why it is not set.

Besides that, set a setter for probot.app which throws an error explaining how to mock authentication with Probot 9+

Describe alternatives you’ve considered

¯\_(ツ)_/¯

Teachability, Documentation, Adoption, Migration Strategy

It’s not a user-facing feature, only an improvement of developer experience which should explain itself with helpful error messages

[gr2m]

PRs welcome 😃

[gr2m]

this should be resolved with latest Probot