Jenkinsfile: run each py.test on a separate executor

Running all py.test processes in parallel on the same executor is not a good idea.
If a node has not much RAM, it could run out of memory when all py.test processes
run at the same time. And If there are only 2 CPU cores, it doesn't make sense
to run more then 2 processes either.
Therefore use a separate executor for each one. The py.test processes will thus
only run in parallel if there are multiple Jenkins executors available.