Add unit tests for ReverseStringUsingStack

Author: navaneedan07

src/test/java/com/thealgorithms/stacks/ReverseStringUsingStackTest.java

@@ -1,8 +1,9 @@
 package com.thealgorithms.stacks;
 
-import org.junit.jupiter.api.Test;
 import static org.junit.jupiter.api.Assertions.*;
 
+import org.junit.jupiter.api.Test;
+
 class ReverseStringUsingStackTest {
 
     @Test