hysteresis: move out of systemlib, move to gtest

This moves the hysteresis test out of the systemlib and makes it its own
small library. Since it still depends on hrt_absolute_time this does not
link yet. My attempt to get all link dependencies together failed.

src/modules/commander/CMakeLists.txt

@@ -62,6 +62,7 @@ px4_add_module(
 		failure_detector
 		git_ecl
 		ecl_geo
+		hysteresis
 	)
 
 if(PX4_TESTING)

src/modules/commander/Commander.hpp

@@ -41,7 +41,7 @@
 #include <lib/mathlib/mathlib.h>
 #include <px4_module.h>
 #include <px4_module_params.h>
-#include <systemlib/hysteresis/hysteresis.h>
+#include <lib/hysteresis/hysteresis.h>
 
 // publications
 #include <uORB/Publication.hpp>