hdf5_ep

[svn-r3818] Purpose:
    Bug fix and clean up.
Description:
    The part that should test 4GB was actually testing 2GB due to
    typo.
Solution:
    Corrected the typo to use 4GB constant.  Rearranged the code
    to group 2GB and 4GB tests in their own.  Removed some duplicated
    testing code.
Platforms tested:
    modi4.