test: split basic HMR test into multiple files (#79064)
Split long-running basic hmr test to parallelize and reduce CI time.
As @huozhi mentioned at https://github.com/vercel/next.js/pull/73613#pullrequestreview-2485717304, the tests won't be separated to `__test__` dir but flattened out on the same level as the fixture dirs.
Port of https://github.com/vercel/next.js/pull/73616, confirmed no new changes.
x-ref: https://github.com/vercel/next.js/commits/canary/test/development/basic/hmr/basic.test.ts
