tests/prime_self_import: Track leaked objects accurately

tests/prime_self_import: Track leaked objects accurately

drm_open_any keeps a buffer handle around for the cleanup sync work,
so we can only grab the buffer count after the latst drm_open_any
call. Otherwise we'll detect a fake leak.

This broke in

commit 2f2c491cf3167befe7c79e4b17afb4f6284dfc84
Author: Mika Kuoppala <mika.kuoppala@intel.com>
Date:   Fri Mar 28 10:52:46 2014 +0200

    lib/drmtest: don't dup quiescent fd

since that additional open drm fd keeps a gem object for the default
context around. Hence why this also only blows up on gen6+ - earlier
platforms don't have hw context support.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79821
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>