Add a test for average (#3084)

Co-authored-by: thanhtri122002 <thanhnt122002@122002>
Co-authored-by: Andrii Siriak <siryaka@gmail.com>

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

@@ -0,0 +1,14 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+
+public class AverageTest {
+    double [] numbers = {3, 6, 9, 12, 15, 18, 21};
+    @Test
+    public void testAverage() {
+        
+        Assertions.assertEquals(12, Average.average(numbers));
+    }
+}