style: fix checkstyle issues (newline and imports)

Author: prashantpiyush1111

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

@@ -1,6 +1,9 @@
 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 static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -68,4 +71,4 @@ void testPeekOnEmptyStack() {
 
 		assertThrows(RuntimeException.class, stack::peek);
 	}
-}
+}