Eliminate a number of memory leaks in tests

[hubertp]

Pull Request Description

Memory usage has spin out of control recently when running our test-suite. Unnecessarily.
This change eliminates a number of leaks that haven't been properly cleaned up on test teardown.
Also closed a number of stale threads.

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• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
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[Akirathan]

Thanks for going over tests one-by-one. We really need to schedule some time for #11683.

[enso-bot]

Hubert Plociniczak reports a new STANDUP for the provided date (2025-03-04):

Progress: Merged fixes for reported NI issues. Profiling test suite and fixing performance issues to reduce CI failures. More testing of NI. It should be finished by 2025-03-05.

Next Day: Next day I will be working on the #12414 task. Continue investigating test suite perf degradation.

[enso-bot]

Hubert Plociniczak reports a new STANDUP for the provided date (2025-03-05):

Progress: Finished perf improvements to our test suite. More testing of NI. Sync regarding telemetry blockers. Did some cleanup in our sbt build #12419. Tried to enable NI in staging. It should be finished by 2025-03-05.

Next Day: Next day I will be working on the #12414 task. Investigate staging issues.

[JaroslavTulach]

Thanks for going over tests one-by-one. We really need to schedule some time for #11683.

It's just about applying this patch - then setup and teardown would be centralized.