Fix test flakiness
Fix test flakiness

Fabricating a post accidentally created an extra user even if the user was
specified.

Example failure in spec/commands/thredded/autofollow_users_spec.rb:16:

https://travis-ci.org/thredded/thredded/jobs/373155461

thredded/thredded,thredded/thredded,jayroh/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded