drm/i915/selftests: add missing gtt shrinker test

Try to catch a bug we've seen in the wild where the shrinker purges the
pd/pdp from under us while allocating our paging structures.

References: https://bugs.freedesktop.org/show_bug.cgi?id=104773
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180131191453.12676-1-matthew.auld@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180131214440.7141-2-chris@chris-wilson.co.uk