Reenable parallelization of packet parsing in packet tests

[rukai]

Ended up down this rabbit hole because I went to remove the num_cpus dep as its now built into rust 1.59, however discovered that we didnt need it at all in the first place.

• Removes commented out code + dependencies used by commented out code
• Reduces test runtime by 10s by parallelizing the workload, theres still 10s of runtime left that could be optimized but I dont have high confidence in the approach of this test in the first place so i'll leave as is.
• Simplifies implementation, rayon is much simpler to use than raw Threadpools