Modified the specpix all apptest such that the output differed from the input...

Modified the specpix all apptest such that the output differed from the input data.  The original test attempted to modify dn values that were not actually in the cube resulting in an output cube identical to the input.  Reference mantis #0399

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4999 41f8697f-d340-4b68-9986-7bafba869bb8