implement test framework

this MR implements the test framework as discussed in our video call and mentioned in #39 (closed). We use a docker-composed based setup to spin up a thredds server that mounts the same volume as the django app such that django can modify the XML-files. The setup is such that first a django app is spin up (via python manage.py runserver), then the thredds-server, then the test suite is ran in another container.

Strictly speaking, the first container that runs python manage.py runserver is not necessary as the django-test container responsible for the test suite anyway creates its own database. but I implemented it anyway in the light of #40, because at one point, the thredds-server should identify himself against the django app and needs a running django app for that

closes #39 (closed)