Avoid potentially unportable parameter substitution in the test scripts.

tests/pio_write.run.in

 #!@SHELL@
 
-prefix=${0##*/}
+prefix=`echo "$0" | sed 's@.*/@@'`
 suffix=${suffix-grb}
 
 if ${mpi_task_num+:} false; then

tests/test_cksum.run.in

 #!@SHELL@
 
-prefix=${0##*/}
+prefix=`echo "$0" | sed 's@.*/@@'`
 suffix=${suffix-grb}
 
 case ${suffix} in

tests/test_cksum_nc_chunk.run.in

 #!@SHELL@
 @ENABLE_NETCDF_FALSE@exit 77
 
-prefix=${0##*/}
+prefix='test_cksum_nc_chunk.run'
 
 ./cksum_write_chunk -b "${prefix}" -m 17 || exit 1
 ./cksum_read "${prefix}.nc" "${prefix}.cksum" || exit 1