Wait for multiple parallel dig commands to fully finish

Wait for multiple parallel dig commands to fully finish

The strategy of running many dig commands in parallel and
waiting for the respective output files to be non empty was
resulting in random test failures, hard to reproduce, where
it was possible that the subsequent reading of the files could
have been failing due to the file's content not being fully flushed.

Instead of checking if output files are non empty, we now wait
for the dig processes to finish.