test: fix flaky benchmark summary

[hi-ogawa]

Description

• follow up to fix(benchmark): fix benchmark summary of single bench suite #5489 (comment)

The test added in the above PR had an assertion of xxx faster than yyy, but it looks quite unstable on CI.

• https://github.com/vitest-dev/vitest/actions/runs/8568796944/job/23483403713#step:9:322
• https://github.com/vitest-dev/vitest/actions/runs/8577751509/job/23511069656?pr=5311#step:10:341
• https://github.com/vitest-dev/vitest/actions/runs/8577739425/job/23511042842?pr=5484#step:9:316

I updated test case to hopefully fix the flakiness.

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