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test: tune down parallelism for some flaky tests

Rodrigo Muino Tomonari requested to merge github/fork/kvakil/tune-less-flaky into main

These tests seem to timeout quite often. I don't know why, but one possible reason is that they are starting a lot of threads. It seems that tests in test/parallel are assumed to only start one thread each, so having 11 threads running at a time feels like a lot.

It also seems that these tests fail in a correlated fashion: take a look at this reliability report. The failures all occur on the same build machines on the same PRs. This suggests to me some sort of CPU contention.

On my Linux machine decreasing the parallelism & iterations here reduce the user time from ~11.5 seconds to ~2 seconds, depending on the test. I have seen these tests take 30-60 seconds on CI (Alpine in particular).

I went back to the diffs that introduced that introduced these changes and verified that they failed at least 90% of the time with the reduced iteration count, which feels sufficient.

Refs: https://github.com/nodejs/node/issues/43499 Refs: https://github.com/nodejs/node/issues/43084

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