Track threads & futures spawned in integration tests to make sure logs are in chronological order

[Gedochao]

X==== Finishing '<test name>' from <test file>

These logs often don't match actual tests ending in integration test logs, as we spawn plenty futures, threads and such which may have not been cleaned up (and may still print to outputs). This is an attempt to track those resources, clean them up after each test and ensure we only print a test has ended when it, in fact, has ended.
At least that's the idea.

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)

How much have your relied on LLM-based tools in this contribution?

extensively

How was the solution tested?

existing tests + a unit test for the tracker (I heard you like tests, so I put unit tests in your integration tests 😐)