Renamed the texture files referenced by test/models/IRRMesh/cellar.irrmesh to the (lowercase) names which are stored in the model file.

This caused an error on case-sensitive operating systems like Linux.

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@457 67173fc5-114c-0410-ac8e-9d2fd5bffc1f