[utils/task-tester] Parallelize submissions reruns

[nrybowski]

Is your feature request related to a problem? Please describe. Currently, all the tested submissions are replayed sequentially. The whole process can be quite slow if multiple hundreds of submissions are considered.

Describe the solution you'd like The task tester should use a thread pool to launch multiple submissions of the same task at the same time. This might speed up the verification process.