Ensure that Rails.env is equal to "test" by default when running tests.

Rails.env when running tests on localhost differs from travis ci
which makes it harder to write tests that check env related things.

railties/test/abstract_unit.rb

+ENV["RAILS_ENV"] ||= "test"
+
 require File.expand_path("../../../load_paths", __FILE__)
 
 require 'stringio'