Add tests for factorial (#3107)

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

@@ -0,0 +1,16 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+
+
+public class FactorialTest {
+
+    @Test
+    public void test() {
+        Factorial fact = new Factorial();
+        assertEquals(120,fact.factorial(5));
+ }
+
+}