Start Integration testcase

This starts the IntegrationTestCase class as described in #4371. While it is incomplete, the parts that are done 'work', and I figured a smaller blob of changes would be easier to discuss and find problems with.
There should be a try catch around this, catching everything except phpunit's own exceptions. Then, all error should be processed by the configured exception renderer. Otherwise testing 500 errors or special API repo see generated by the renderer  would not be possible