Skip MPI tests if MPI_LAUNCH is not available.

tests/pio_write_deco2d_run.in

 #!@SHELL@
+test 'x@MPI_LAUNCH@' != xtrue || exit 77
 
 LOG="${LOG-pio_write_deco2d.log}"
 mpi_task_num="${mpi_task_num-6}"

tests/pio_write_run.in

 #!@SHELL@
+test 'x@MPI_LAUNCH@' != xtrue || exit 77
+
 LOG=${LOG-pio_write.log}
 mpi_task_num="${mpi_task_num-4}"
 suffix="${suffix-grb}"