Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

tests/core/io/test_image.h

@@ -300,4 +300,5 @@ TEST_CASE("[Image] Modifying pixels of an image") {
 			"flip_y() should not leave old pixels behind.");
 }
 } // namespace TestImage
+
 #endif // TEST_IMAGE_H

tests/core/io/test_resource.h

@@ -28,8 +28,8 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#ifndef TEST_RESOURCE
-#define TEST_RESOURCE
+#ifndef TEST_RESOURCE_H
+#define TEST_RESOURCE_H
 
 #include "core/io/resource.h"
 #include "core/io/resource_loader.h"
@@ -111,4 +111,4 @@ TEST_CASE("[Resource] Saving and loading") {
 }
 } // namespace TestResource
 
-#endif // TEST_RESOURCE
+#endif // TEST_RESOURCE_H