Add Test Examples

[type1fool]

Problem Statement

I am attempting to write tests for a controller that uses the Ueberauth plug and implements a callback/2 function. After digging through documentation for Ueberauth, Plug, and Phoenix, and after reviewing the code in ueberauth_example I do not see a clear path for testing the function in my application.

The google_test.exs in ueberauth_google has a significant amount of setup and other code that may not be necessary in an application using the package.

Solution Brainstorm

The example repo does a great job at illustrating implementation of Ueberauth with various providers. Please add example tests so it is also clear how to make sure things work correctly.