Cleanup: Cycles: Use gtest header instead of Blender test header

To avoid conflicts with glog logging macros, and because there is just no
need to have this dependency.

Pull Request: https://projects.blender.org/blender/blender/pulls/143719

intern/cycles/test/util_math_float3_test.cpp

@@ -5,9 +5,9 @@
 /* Note: These fixtures test default micro-architecture optimization defined in the
  * util/optimization.h. */
 
-#include "testing/testing.h"
+#include <gtest/gtest.h>
+
 #include "util/math.h"
-#include "util/system.h"
 
 CCL_NAMESPACE_BEGIN