Refactor Pallas MPMD tests into a dedicated file.
This change moves the `tpu_async_sc_tc_test` and the `MpmdMapTest` class from `tpu_sparsecore_pallas_test.py` into a new file, `tpu_pallas_mpmd_test.py`. The tests are updated to use `pl.kernel` instead of `mpmd.mpmd_map`.
PiperOrigin-RevId: 907289594
