lib/core: Add optional reason for timeout failure

"Timed out" isn't a terribly informative message, allow users to set
something more informative. Inspired by a request from Jesse.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

lib/tests/igt_timeout.c

@@ -3,6 +3,6 @@
 
 igt_simple_main
 {
-	igt_set_timeout(1);
+	igt_set_timeout(1, "Testcase");
 	sleep(5);
 }