tests: fix duplicate PostVersionTest class.

Fix `PostVersionTest` being defined in two different places, which broke
the test runner if you tried to run the system tests at the same time as
the regular tests.

test/system/post_version_test.rb

@@ -1,6 +1,6 @@
 require "application_system_test_case"
 
-class PostVersionTest < ApplicationSystemTestCase
+class PostVersionSystemTest < ApplicationSystemTestCase
   context "Post versions" do
     setup do
       @user = create(:builder_user)