clean up tests

- set up integration test for Snakefile in dedicated folder; current test case was left untouched for the time being, despite requiring large input files
- set up DAG chart creation test in dedicated folder; script creates an SVG representation of the workflow DAG at `images/workflow_dag.svg`
- both tests have been added to the GitLab CI/CD configuration; the latter test ensures that always the latest version of the
- all tests are now located inside subdirectories of `tests/`; test scripts and configuration files for test runs etc. have been moved to the appropriate test directories
- for the time being, required input files for each test are placed within the individual test directories; a layout for common test files should be introduced later and paths and bind paths in tests adapted
- make script `scripts/labkey_api.py` executable