Fixed most of the rest tests and deleted __str__ with using_template.

Created by: VinczeRobert

About the tests:

1. I added a fixture which creates a flask app and used it with test_context to solve the out of context problem which appeared in several tests.
2. test_server_get_operations_for_datatype failes because result has only 2 elements and it should have more than 3
3. If you debug test_server_fire_simulation it says that the path is not a zip file even though to me it seems like it is. The file actually exists at the specified path and it has .zip extension.
4. test_get_projects doesn't fail if you run it separately. (in group it says that it doesn't have permission to do the teardown method)
5. I deleted the test test_get_project_invalid_username because the retrieving of projects is not done by username anymore. We should probably write a test that replaces this.
6. test_get_users says that it has empty keycloak config path.