[Piglit] Use config file for external suite locations v2

[Dylan Baker]

This is a v2 of a series I sent out previously, it changes external
suite integration to use piglit.conf instead of symlinks in the bin
directory. It also adds support for putting piglit.conf files in more
persistent locations, namely $HOME/.config or $XDG_CONFIG_HOME. With
these two changes it is possible for a configuration that doesn't get
deleted by git clean.

V2 corrects a number of issues Chad had with the original series, and
adds some unit tests for the changes.