Add unit test for SulfurasItem immutability

Java/src/test/java/com/gildedrose/SulfurasItemTest.java

@@ -0,0 +1,16 @@
+package com.gildedrose;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class SulfurasItemTest {
+
+    @Test
+    void sulfurasNeverChanges() {
+        Item item = new Item("Sulfuras, Hand of Ragnaros", 0, 80);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(0, item.sellIn);
+        assertEquals(80, item.quality);
+    }
+}