tests: fix null uploader_ip_addr exceptions in create(:post).

Caused by a change in FactoryBot 5. Associations in factories are now
constructed using the same strategy as the base object, meaning that
using `build` to construct an object will also construct the
associations using `build`. This meant that overriding `create` to do
`build` + `save` broke the way that associations were constructed.

https://github.com/thoughtbot/factory_bot/blob/master/GETTING_STARTED.md#associations

test/factories/user.rb

@@ -1,5 +1,5 @@
 FactoryBot.define do
-  factory(:user, aliases: [:creator, :updater]) do
+  factory(:user, aliases: [:creator, :updater, :uploader]) do
     sequence :name do |n|
       "user#{n}"
     end