Fix trade activity tests: use a valid item fixture, not a placeholder

In 540ce08caa, I updated the Item class
to be more explicit about what fields are required, so this test would
fail in a more helpful way, instead of just crashing from `name` being
`nil` when trying to infer the Dyeworks info.

Now, we update the test to use Rails's standard "fixture" system to set
up a more-correct placeholder item, instead!

test/fixtures/items.yml

@@ -0,0 +1,14 @@
+straw_hat:
+  id: 58
+  name: Straw Hat
+  description: "This straw hat will keep the sun out of your pets eyes in
+    bright sunlight."
+  thumbnail_url: https://images.neopets.com/items/straw-hat.gif
+  type: Clothes
+  category: Clothes
+  rarity: Very Rare
+  rarity_index: 90
+  price: 376
+  weight_lbs: 1
+  zones_restrict: 0000000000000000000000000001000000001010000000000000
+  species_support_ids: "35"

test/trade_activity_test.rb

@@ -203,12 +203,6 @@ class TradeActivityTest < ActiveSupport::TestCase
 
 	setup do
 		freeze_time # to compare timestamps accurately
-
-		Item.create!(
-			thumbnail_url: "https://images.neopets.com/foo.png",
-			zones_restrict: "",
-			price: 123,
-		)
 	end
 
 	private