Add unit tests for ReverseStringUsingStack

Author: navaneedan07

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

@@ -1,6 +1,8 @@
 package com.thealgorithms.stacks;
 
-import static org.junit.jupiter.api.Assertions.*;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;