Add test structure for InvoiceLineItem

This doesn't come back directly from the API so the suite is empty, but
just for completeness add a test file for the newly created
`InvoiceLineItem` model.

test/stripe/invoice_line_item_test.rb

@@ -0,0 +1,7 @@
+require File.expand_path('../../test_helper', __FILE__)
+
+module Stripe
+  class InvoiceLineItemTest < Test::Unit::TestCase
+    FIXTURE = API_FIXTURES.fetch(:invoice_line_item)
+  end
+end