test: simplify browser mode tests

[hi-ogawa]

Description

Extracted a change from #4878

I split this test case into a separate vitest root directory and rewrote an assertion based on stdout/stderr with startVitest API, so it might be easier to debug.

I'm still not sure why the previous approach is failing in some cases in my new PRs, but I thought this simplification would make sense either way.

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[hi-ogawa]

Hmm, now main test runner.test.mjs is failing and the log is indicating no tests are found:

https://github.com/vitest-dev/vitest/actions/runs/7418249822/job/20185934235?pr=4879#step:11:72

[hi-ogawa]

After doing random attempt to simplify the minimal repro, it looks like combining diff and optimizeDeps is causing some flakiness on webkit.

Still doing a dirty debugging, but one thing I noticed is that when switching browser test directory by --root option, Vite's cache directory doesn't change, so it looks like it's causing pre-bundling whenever changing --root since old pre-bundling in test/browser/node_modules/.vite/deps is always invalidated.

This can be observed by:

# first run without --root
$ DEBUG=vite:deps pnpm test-fixtures 

# 2nd run with --root
$ DEBUG=vite:deps pnpm test-fixtures --root fixtures/custom-base

vite:deps removing old cache dir /home/hiroshi/code/others/vitest/test/browser/node_modules/.vite/deps
...

---

Also I finally got Playwright/Webkit setup on my PC and I confirmed the same issue happens locally as well (though it's not consistent). The error log is not consistent either, but sometimes I get this log:

---- Unhandled Errors

Vitest caught 1 unhandled error during the test run.
This might cause false positive tests. Resolve unhandled errors to make sure your tests are not affected.

--- Unhandled Rejection
TypeError: Importing a module script failed.
----

 Test Files  no tests
      Tests  no tests
     Errors  1 error
   Start at  17:38:46
   Duration  6.43s (transform 0ms, setup 0ms, collect 0ms, tests 0ms, environment 0ms, prepare 0ms)


 FAIL  Tests failed. Watching for file changes...
       press h to show help, press q to quit
 ✓ test/basic.test.ts (4)
   ✓ basic
   ✓ basic 2
   ✓ x is x
   ✓ y is x

[hi-ogawa]

I don't think this simplification particularly matters, so I'll close this.

This PR ended up with me investigating Webkit flakiness and I made a simpler repro in a separate PR #4908.