Add More Tests (#4148)

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

@@ -0,0 +1,28 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class LeonardoNumberTest {
+    @Test
+    void leonardoNumberNegative() {
+        assertThrows(ArithmeticException.class, ()-> LeonardoNumber.leonardoNumber(-1));
+    }
+    @Test
+    void leonardoNumberZero() {
+        assertEquals(1, LeonardoNumber.leonardoNumber(0));
+    }
+    @Test
+    void leonardoNumberOne() {
+        assertEquals(1, LeonardoNumber.leonardoNumber(1));
+    }
+    @Test
+    void leonardoNumberFive() {
+        assertEquals(15, LeonardoNumber.leonardoNumber(5));
+    }
+    @Test
+    void leonardoNumberTwenty() {
+        assertEquals(21891 , LeonardoNumber.leonardoNumber(20));
+    }
+}