Add tests for sum of digits (#3083)

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

@@ -0,0 +1,25 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * @author SirFixalot16
+ * @since 01/06/22
+ */
+public class SumOfDigitsTest {
+    @Test
+    void isSumOf2Digits() {
+        SumOfDigits sum = new SumOfDigits();
+        assertEquals(11, sum.sumOfDigits(56));
+    }
+    void isSumOf3Digits() {
+        SumOfDigits sum = new SumOfDigits();
+        assertEquals(12, sum.sumOfDigits(192));
+    }
+    void isSumOf4Digits() {
+        SumOfDigits sum = new SumOfDigits();
+        assertEquals(25, sum.sumOfDigits(8962));
+    }
+}