Move UnitTestVTKDataSetReader files to git-lfs

The UnitTestVTKDataSetReader test reads in .vtk files of multiple formats. The origins of this test predate the use of git-lfs and a data directory, so the test defined in the source code several strings containing the contents of the files.

That is no longer necessary. This change removes those strings from the file and creates actual files in the data directory. This is easier to manage as well as more convenient if you want to use the same data in a different test or just take a look at the data in a different program (such as VisIt or ParaView).

I am also hoping this clears up some intermittent failures of UnitTestVTKDataSetReader. (Perhaps the constant writing and reading of files is getting fouled up in the container.)