fix: set MPICH_GPU_SUPPORT_ENABLED for GPU test dispatch

The Frontier template sets MPICH_GPU_SUPPORT_ENABLED=1 for GPU runs,
but this was outside mpi_config so the direct test runner never set
it. Without it, GPU-aware MPI (RDMA) cannot handle device pointers,
causing Bus errors on multi-rank GPU tests.

Author: Spencer Bryngelson

toolchain/mfc/test/case.py

@@ -309,6 +309,7 @@ def run(self, targets: List[Union[str, MFCTarget]], gpus: Set[int]) -> subproces
             gpu_ids = ",".join(str(g) for g in gpus)
             env["CUDA_VISIBLE_DEVICES"] = gpu_ids
             env["HIP_VISIBLE_DEVICES"] = gpu_ids
+            env["MPICH_GPU_SUPPORT_ENABLED"] = "1"
 
         # Resolve binary paths using the original (unmodified) params for slug
         slug_case = self.to_input_file()