tests: remove SKIP_CLOUDFLARE_CHECK env variable.

This was a hack to deal with the Cloudflare check sometimes being slow
or timing out during tests. The call to https://api.cloudflare.com/client/v4/ips
could hang if there were IPv6 connectivity problems. If this happens, make
sure that IPv6 is configured properly and that `curl -v --http1.1 -6 https://api.cloudflare.com/client/v4/ips`
works.

test/unit/downloads/file_test.rb

@@ -4,15 +4,10 @@ module Downloads
   class FileTest < ActiveSupport::TestCase
     context "A post download" do
       setup do
-        ENV["SKIP_CLOUDFLARE_CHECK"] = "1"
         @source = "http://www.google.com/intl/en_ALL/images/logo.gif"
         @download = Downloads::File.new(@source)
       end
 
-      teardown do
-        ENV["SKIP_CLOUDFLARE_CHECK"] = nil
-      end
-
       context "for a banned IP" do
         setup do
           Resolv.expects(:getaddress).returns("127.0.0.1").at_least_once