Don't consider directories as executables in find_exec
Don't change directory when invoking snogray

snogray is now able to be run from any directory, so don't bother
changing the current directory when running it.

This also means we don't need to make filenames passed to snogray
absolute.
Add "clamp_output" test-parameter

This will clamp output files before comparing them, etc, which can be
useful in some case for very bright and small lights that otherwise
dominate the overall intensity of the scene, and can result in a lot
of variation at low sample densities.
Get rid of subshell in run_snogray

There's no need for it now that we're not changing the current
directory.