It seems in the distant past there was a "hsmdfour" binary, which was then
moved to a "sum" argument to rdiff, that was then removed. There also used to
be an mdfour.test test script. All of these are long gone, but the test data
lived on until now.

This looks like it was once one of the main rdiff test scripts, but it is
nolonger used and other tests provide better coverage.