itemstats: Add tooltip string method tests

This commit adds tests to ensure proper formatting of the tooltip string
so as to prevent bugs like the one fixed by 7ca4a929.

runelite-client/src/main/java/net/runelite/client/plugins/itemstats/ItemStatOverlay.java

@@ -236,7 +236,8 @@ public class ItemStatOverlay extends Overlay
 		return b.toString();
 	}
 
-	private String buildStatBonusString(ItemStats s)
+	@VisibleForTesting
+	String buildStatBonusString(ItemStats s)
 	{
 		ItemStats other = null;
 		final ItemEquipmentStats currentEquipment = s.getEquipment();