[ENHANCEMENT] Investigate speeding up playwright test suite in CI

It currently takes some [time](https://github.com/GoogleChrome/webstatus.dev/pull/2308#pullrequestreview-3906215552) to run the playwright tests. However, we could benefit from some optimizations.
- [Sharding](https://github.com/GoogleChrome/webstatus.dev/pull/2308#pullrequestreview-3906215552), but then we would need to modify download report make [target](https://github.com/GoogleChrome/webstatus.dev/blob/b3ea385973519af5ff28b199469e859c9e37bfba/Makefile#L552-L555) to do merging of the reports as documented in the playwright docs.