Add unit tests for NormalItem quality and sellIn behavior

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

@@ -0,0 +1,33 @@
+package com.gildedrose;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class NormalItemTest {
+
+    @Test
+    void normalItemDegradesByOneBeforeSellIn() {
+        Item item = new Item("Normal Item", 5, 10);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(4, item.sellIn);
+        assertEquals(9, item.quality);
+    }
+
+    @Test
+    void normalItemDegradesTwiceAfterSellIn() {
+        Item item = new Item("Normal Item", 0, 10);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(-1, item.sellIn);
+        assertEquals(8, item.quality);
+    }
+
+    @Test
+    void qualityNeverNegative() {
+        Item item = new Item("Normal Item", 0, 0);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(0, item.quality);
+    }
+}