test: split fixtures for browser test

[hi-ogawa]

Description

Base branch: #4867 (new change is only the last commit)

Testing to see if splitting the custom base test would affect test failure on CI.
I also rewrote an assertion to be based on stdout/stderr, so it might be easier to debug.

I'll retry CI for a few times by force-pushing the same commit.

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[AriPerkkio]

Was this using npx before? Signal terminals are buggy when scripts are launched via npx. I can't find the link for the issue or older references for these issues right now. Consider using startVitest instead.

[AriPerkkio]

pnpm exec should also work correctly.

[hi-ogawa]

Currently browser mode testing is written with execa + npx vitest, so I was keeping the same structure from:

vitest/test/browser/specs/run-vitest.mjs

Lines 7 to 12 in c946b1c

	const argv = ['vitest', '--run', `--browser.name=${browser}`]
	if (browser !== 'safari')
	argv.push('--browser.headless')
	const { stderr, stdout } = await execa('npx', argv.concat(moreArgs), {

The test runner used here is node --test, so test utils like runVitest are not available and runner.test.mjs would probably be easier to written with execa, so I'll keep that there for now.

This small test custom-base.test.mjs should be fine with startVitest API, so I'll rewrite that. Thanks for the suggestion!

---

Btw, I found things are already flaky without update-snapshot.test.mjs from #4867, so I'm now testing with only runner.test.mjs and custom-base.test.mjs here #4879.

[hi-ogawa]

Closing this in favor of the other one

• test: simplify browser mode tests #4879