test/mpi/threads/spawn: initialize the MTest framework

th_taskmaster.c uses MTest functions for forking and joining threads, but the
MTtest framework is uninitialized, which can cause an uninitialized error.  This
patch fixes it.

test/mpi/threads/spawn/th_taskmaster.c

@@ -68,13 +68,13 @@ int main(int argc, char *argv[])
 #endif /* USE_THREADS */
 
 #ifdef USE_THREADS
-    CHECK_SUCCESS(MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided));
+    CHECK_SUCCESS(MTest_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided));
     if (provided != MPI_THREAD_MULTIPLE) {
         fprintf(stderr, "MPI does not provide THREAD_MULTIPLE support\n");
         MPI_Abort(MPI_COMM_WORLD, -1);
     }
 #else
-    CHECK_SUCCESS(MPI_Init(&argc, &argv));
+    CHECK_SUCCESS(MTest_Init(&argc, &argv));
 #endif
 
     CHECK_SUCCESS(MPI_Comm_get_parent(&parent));
@@ -141,7 +141,7 @@ int main(int argc, char *argv[])
     }
 
   fn_exit:
-    MPI_Finalize();
+    MTest_Finalize();
 
     return 0;
 }