Add a selftest to exercise evicting neighbouring nodes that conflict due
to page colouring in the GTT.

v2: add a peppering of comments
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170306235414.23407-4-matthew.auld@intel.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>