test: add unit tests for StackUsingLinkedList

Author: tarandeepsingh-wd

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

@@ -49,4 +49,10 @@ public void testSize() {
     public void testPopOnEmptyStack() {
         assertThrows(RuntimeException.class, () -> stack.pop());
     }
+    @Test
+    public void testPeekOnEmptyStackThrowsException() {
+        RuntimeException exception =
+                assertThrows(RuntimeException.class, () -> stack.peek());
+        assertEquals("Stack is empty", exception.getMessage());
+    }
 }