tests compare against git hash, eliminate cached .h5

This updates the test infrastructure to store a git hash of the reference
code, instead of generated .h5 files.  The test infrastructure creates a
cache of the reference .h5 files from the specified git hash if it doesn't
already exist, and then compares against that cache.

This gets rid of the need for git-lfs and the cached .h5 files, requiring
instead the code is checked out from git.