(#10) add radial gradient unit tests, add missing opens in module-info

src/main/java/tech/fastj/graphics/util/gradients/Gradients.java

@@ -75,7 +75,7 @@ static void sameBoundaryCheck(Boundary start, Boundary end) {
 
     static void minimumRadiusValueCheck(float radius) {
         if (radius <= 0f) {
-            throw new IllegalStateException("The radius for a gradient must be larger than 0.");
+            throw new IllegalArgumentException("The radius for a gradient must be larger than 0.");
         }
     }
 

src/test/java/module-info.java

@@ -5,6 +5,7 @@
     opens unittest.testcases.graphics to org.junit.platform.commons;
     opens unittest.testcases.graphics.game to org.junit.platform.commons;
     opens unittest.testcases.graphics.util to org.junit.platform.commons;
+    opens unittest.testcases.graphics.util.gradients to org.junit.platform.commons;
 
     opens unittest.testcases.math to org.junit.platform.commons;
 

src/test/java/unittest/mock/graphics/MockBoundariesDrawable.java

@@ -0,0 +1,17 @@
+package unittest.mock.graphics;
+
+import tech.fastj.graphics.Drawable;
+import tech.fastj.graphics.util.DrawUtil;
+
+import tech.fastj.systems.control.Scene;
+
+public class MockBoundariesDrawable extends Drawable {
+
+    public MockBoundariesDrawable() {
+        setBounds(DrawUtil.createBox(0.0f, 0.0f, 1.0f));
+    }
+
+    @Override
+    public void destroy(Scene originScene) {
+    }
+}