Parallelize remote file downloads within each iree_tests directory.

[ScottTodd]

Progress on https://github.com/nod-ai/SHARK-TestSuite/issues/285

This is a simple improvement over serial processing, but it could still be improved further.

Looks like this shaves ~10 seconds off runs in this repo:

• Before 45s: https://github.com/nod-ai/SHARK-TestSuite/actions/runs/9996869262/job/27632145027#step:6:15
• After 35s: https://github.com/nod-ai/SHARK-TestSuite/actions/runs/9997455742/job/27634060037?pr=297#step:6:15

I saw 2m+ runs in IREE, hopefully this helps there too. Should be able to get the total time down to 10-20 seconds.

[saienduri]

Nice, hopefully this parallelization doesn't crash anything lol