[Bug]: JSON reporter emits a separate spec per project when testDir is not the current directory

Finding: In Playwright Test 1.64.0 on Node 22.22.0 and Ubuntu 24.04 (as reported), running npx playwright test --reporter=json from the project root with testDir set to ./tests and two projects (chromium, firefox) reportedly yields one spec per project instead of one merged spec holding both projects' tests ([issue #43266](https://github.com/microsoft/playwright/issues/43266)). Reported behavior: The reporter's config uses testDir './tests' and two projects, with a single trivial test file. From the project root the JSON output lists the 'has title' spec twice, once with chromium and once with firefox. Tests inside test.describe are duplicated the same way. Running from inside tests/ with -c .. reportedly gives the expected single spec containing both projects. The report shape therefore depends on the directory the command is run from ([issue #43266](https://github.com/microsoft/playwright/issues/43266)). Suspected cause (hypothesis, not confirmed): The reporter points to _mergeTestsFromSuite in packages/playwright/src/reporters/json.ts. It reportedly compares s.file against toPosixPath(path.relative(rootDir, spec.file)). Since spec.file is already relative to rootDir, path.relative resolves it against process.cwd(), so the comparison would only succeed when cwd equals rootDir. The reporter notes that suites in the same function use _locationMatches, which could also work for specs. The reporter also guesses the regression dates to 1.27, when the comparison changed from an absolute test.location.file to the relative spec.file. All of this is the reporter's own analysis, and no maintainer has confirmed it ([issue #43266](https://github.com/microsoft/playwright/issues/43266)). Practical implication: Consumers that post-process JSON report output, for example by grouping by spec title, may see different structures depending on cwd when testDir is not the cwd. A reporter-described workaround is to run from inside the test directory with -c pointing to the config, which reportedly produces the merged shape. This is unverified by KFA, and it may change relative path resolution for other tooling. Status: At the time of the supplied data the issue is open with no comments, no maintainer response, no linked fix commit and no released fix. The reporter offers to submit a PR with a regression test in tests/playwright-test/reporter-json.spec.ts, but no PR was supplied. Limitation: This is a single external report. It was not reproduced by KFA, and the supplied comment coverage was unavailable because of an HTTPError, so later discussion may exist that was not seen.