Add a test for VampireNumb.splitIntoSortedDigits

Author: Shisoik

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

@@ -3,8 +3,8 @@
 import org.junit.jupiter.api.Test;
 
 import static com.thealgorithms.maths.VampireNumber.isVampireNumber;
-import static org.junit.jupiter.api.Assertions.assertFalse;
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import static com.thealgorithms.maths.VampireNumber.splitIntoSortedDigits;
+import static org.junit.jupiter.api.Assertions.*;
 
 class VampireNumberTest {
     @Test
@@ -26,4 +26,10 @@ void areNotVampireNumbers() {
         assertFalse(isVampireNumber(51, 39, false));
         assertFalse(isVampireNumber(51, 39, true));
     }
+
+    @Test
+    void testSplitIntoSortedDigits() {
+        assertEquals("123", splitIntoSortedDigits(321));
+        assertEquals("02234", splitIntoSortedDigits(20, 324));
+    }
 }