tests: free pixman image in yuv-buffer-test

Fixes all ASan reported leaks in this test when using AMD Mesa driver.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

tests/yuv-buffer-test.c

@@ -489,5 +489,6 @@ TEST_P(yuv_buffer_shm, yuv_cases)
 	assert(match);
 
 	yuv_buffer_destroy(buf);
+	pixman_image_unref(img);
 	client_destroy(client);
 }