chore(CI): Split basepath test suite into smaller and more parallel test suites (#78892)
Same basic idea as https://github.com/vercel/next.js/pull/78787
This test suite has a lot of timeouts with rspack, which causes it to take a long time to run (though not quite long enough that it breaks the job).
In general, large test suites are good to break up, because they benefit from parallelism.