Add file property to JUnit report entry

[dfrankland]

Many tools look for the file attribute as a way of understanding where a test is located and attributing some statistics or codeowners.

[yury-s]

Please start with filing a bug and describing the problem. Also the change will need a test if we want it to be landed.

[yury-s]

A couple of concerns on the semantics of file if this lands:

1. Base-dir convention. suiteName here is path.relative(config.rootDir, file) (set in testLoader.ts). Other ecosystems disagree on what file should be anchored to — pytest uses the invocation dir, several Jest/Vitest reporters use CWD, some emit absolute paths. Tools consuming JUnit in CI (GitHub Actions annotators, Codecov path-fixing, CODEOWNERS mappers, test-insight products like Trunk/Datadog/BuildPulse) typically resolve file against the repo root or CWD, not Playwright's rootDir. In monorepos where the config lives in a subfolder this silently produces paths that don't match the git tree and annotations get dropped or misattributed.

2. Path separators. On Windows, path.relative returns \-separated paths; most downstream tools match against forward-slash git paths.

Today classname already carries the same value, so nothing behaviorally changes — but once file is the attribute consumers prefer, the rootDir-relative + native-separator convention becomes load-bearing. Worth either normalizing to forward slashes + anchoring to something git-root-ish, or documenting the contract explicitly.