Add unit tests for BackstageItem quality increase and expiration

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

@@ -0,0 +1,39 @@
+package com.gildedrose;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class BackstageItemTest {
+
+    @Test
+    void backstageIncreasesByOneOver10Days() {
+        Item item = new Item("Backstage passes to a TAFKAL80ETC concert", 15, 20);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(21, item.quality);
+    }
+
+    @Test
+    void backstageIncreasesByTwoBetween6And10Days() {
+        Item item = new Item("Backstage passes to a TAFKAL80ETC concert", 10, 20);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(22, item.quality);
+    }
+
+    @Test
+    void backstageIncreasesByThreeBetween1And5Days() {
+        Item item = new Item("Backstage passes to a TAFKAL80ETC concert", 5, 20);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(23, item.quality);
+    }
+
+    @Test
+    void backstageQualityDropsToZeroAfterConcert() {
+        Item item = new Item("Backstage passes to a TAFKAL80ETC concert", 0, 20);
+        GildedRose app = new GildedRose(new Item[]{item});
+        app.updateQuality();
+        assertEquals(0, item.quality);
+    }
+}