The new example 31 produces a minimal plot file so include that example in the diff test.
The new example 31 produces a minimal plot file so include that example
in the diff test.

svn path=/trunk/; revision=9126

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot