[frontend] more robust ./run_tests.sh
    
    - reuse it in copr-frontend.spec
    - reuse './manage.py tests'
    - add --coverage option into manage.py
    - wait till redis shuts down
    - do 'set -e' to fail asap, and 'set -x' to see logs
    - install 'cleanup' trap to always kill redis
    - put copr_common on PYTHONPATH
    
    Fixes: #415