[Piglit] Use more piglit.conf

[Dylan Baker]

This series makes more use of piglit.conf and less use of symlinks.
Why? symlinks are aweful. git clean? re-symlink. out of tree build?
re-symlink. lots of other cases? re-symlink.

This series makes working with piglit.conf much nicer since it can be
put in three locations 1) . 2) in the root of the piglit dir (lib/piglit
when installed), or 3) in the XDG_CONFIG_HOME dir ($HOME/.config). This
means that piglit.conf can survive across all of the above situations.