test: reduce usage of execa, add runVitest test utils

[AriPerkkio]

Using execa on CI is slow and flaky. This PR removes usage of execa from every test where it's possible and uses startVitest Node API instead. This is done by adding test/test-utils package which provides centralized abstractions for running startVitest. As some of the tests still require running Vitest through CLI, there is also utility method runVitestCli which is a simple abstraction over execa and supports interactive watch-mode simulation.

Goal is to make CI stable and faster, and to not have multiple duplicated implementations for running npx vitest in test files/projects. When adding new tests the runVitest should be used by default. If CLI usage is explicitly required, use runVitestCli instead.

There's quite much changes so I've rebased the commits clearly for possible commit-by-commit review.

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[sheremet-va]

Not sure about speed improvements. CI from #3430 is faster 🤔

[AriPerkkio]

The CI speed seems to vary so maybe re-run helps. Mostly I hope these will bring some stability to those flaky execa + Windows test runs.

These results I got on my fork:

And these are the comparison I did locally:

• test/bail: PR 8.81s, main 13.07s 🚀
• test/benchmark: PR 4.8s, main 5.5s
• test/config: PR 0.8s, main 2.3s 🚀
• test/fails: PR 2.5s, main 7.1s 🚀
• test/filters: PR 1s, main 1s
• test/global-setup: PR 2.3s, main 2.3s
• test/global-setup-fail: PR 0.4s, main 0.6s
• test/related: PR 1.5s, main 1.4s ⚠️
• test/reporters: PR 4.9s, main 6s
• test/shard: PR 1.1s, main 1.8s
• test/stacktraces: PR 2s, main 6s 🚀
• test/typescript: PR 2s, main 2.3s
• test/ui: PR 2s, main 2.3s
• test/watch: PR 17s, main 17s