tests: add gem_unfence_active_buffers

Unfortunately this requires slab poisoning to catch anything :(

Also add a new helper to drmtest to get the available fence count.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 9ec07c6..369b09a 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -54,6 +54,7 @@
 	gem_double_irq_loop \
 	gem_ring_sync_loop \
 	gem_pipe_control_store_loop \
+	gem_unfence_active_buffers \
 	gem_unref_active_buffers \
 	gem_reloc_vs_gpu \
 	drm_vma_limiter \