Skip 'pio_cksum_writer' when '--disable-grib'.

tests/pio_cksum_writer.in

 #!@SHELL@
 @USE_MPI_FALSE@exit 77
+test 'x@ENABLE_GRIB@' = xyes || exit 77
 
 pio_write_args="-p PIO_WRITER -w 4"
 mpi_task_num=8