test: use startVitest instead of execa

[AriPerkkio]

Description

Attempts to fix flaky CI. Replaces usage of runVitestCli() with lighter runVitest() utility. This reduces the amount of node:child_process launched during tests (via execa). The runVitestCli() should only be used when tests require interactive watch mode or are explicitly testing CLI options.

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[hi-ogawa]

Nice, thanks a lot!