Moved JSON API Responses to test fixtures

This PR moves the JSON API responses used in the tests to fixture files
within the tests folder. This allows reuse and the ability to lint/validate
these fixtures if required - although not covered in this PR.

I've added TestCase::getPsr7StreamForFixture and TestCase::getPsr7JsonResponseForFixture
to reduce code duplication and enable some assertions around the expected fixures/responses.

Thanks,

tests/Auth/APIKeyTest.php

@@ -5,7 +5,7 @@
  * Date: 13/01/2017
  * Time: 17:15
  */
-class APIKeyTest extends PHPUnit_Framework_TestCase
+class APIKeyTest extends TestCase
 {
     public function testGetHeaders()
     {