Add Tests for Dudeney Number (#3336)

src/test/java/com/thealgorithms/maths/DudeneyNumberTest.java

@@ -0,0 +1,18 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class DudeneyNumberTest {
+
+    @Test
+    void isDudeney() {
+        final int validDudeneyNumber = 512;
+        final int invalidDudeneyNumber = 125;
+
+        assertTrue(() -> DudeneyNumber.isDudeney(validDudeneyNumber));
+        assertFalse(() -> DudeneyNumber.isDudeney(invalidDudeneyNumber));
+
+    }
+}