test: fix flaky slowTestThreshold test

[hi-ogawa]

Description

On CI, it sometimes happens that the "fast" one (10 ms) is taking over slowTestThreshold (100 ms) possibly due to a heavy cpu load.

https://github.com/vitest-dev/vitest/actions/runs/7808613959/job/21299124920?pr=5093#step:8:811

I tweaked some numbers so that hopefully it will be less flaky.

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