test: set PYTHONPATH when running the compliance tool

By setting the PYTHONPATH when running the compliance tool in the tests,
it is no longer required to manually set PYTHONPATH when running the
tests with `python -m unittest` or `coverage`. Thus don't set PYTHONPATH
in the CI script anymore.
Furthermore abstract running the compliance tool with a function,
eliminating a lot of duplicate code.

Author: jkhsjdhjs

.github/workflows/ci.yml

@@ -38,9 +38,7 @@ jobs:
       run: |
         python test/_helper/setup_testdb.py -u "admin" -p "$COUCHDB_ADMIN_PASSWORD"
     - name: Test with coverage + unittest
-      # Add source directory to PYTHONPATH to allow testing our CLI scripts, which import our modules
       run: |
-        export PYTHONPATH=".:$PYTHONPATH"
         coverage run --source=basyx -m unittest
     - name: Report test coverage
       if: ${{ always() }}