vol-tests can't be built without szip when HDF5_VOL_TEST_ENABLE_PARALLEL is on.

[hyoklee]

@jhendersonHDF , is SZIP required for parallel test?
If so, please update README.md.

 >> 239    /tmp/hyoklee/spack-stage/spack-stage-hdf5-vol-tests-hyoklee.master-\
hnzcu35euaqbzxmya44fqs7qemdk23si/spack-src/hdf5_testpar/t_filter_read.c:26:22: \
fatal error: szlib.h: No such file or directory
     240     #   include "szlib.h"

[jhendersonHDF]

No, SZIP is not required for the parallel tests to be built. All code that uses szip-related calls is surrounded by

#ifdef H5_HAVE_FILTER_SZIP

and will not be built unless the HDF5 installation reports that SZIP is available. This suggests that there is a problem with the HDF5 installation being used.