Exclude benchmark tests by default, add --benchmarks flag (#55725)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55725

changelog: [internal]

By default, `yarn fantom` now excludes benchmark tests (`*-benchmark-itest.js` files) to speed up regular test runs.

Benchmark tests are typically slower and used for performance analysis rather than correctness testing. Excluding them by default improves the developer experience for everyday test runs.

To run benchmark tests, use the new `--benchmarks` flag:
```
yarn fantom --benchmarks
```

Reviewed By: andrewdacenko

Differential Revision: D92150163

fbshipit-source-id: 4c40ef1ba542c4856221373d8239ba603e8f6620