Fix @artist.ban! test case.

Must use an admin in this test now because admin privileges are needed
to create the `banned_artist` tag implication.

(fixes regression in 7e3284c)

test/unit/post_test.rb

@@ -478,8 +478,10 @@ class PostTest < ActiveSupport::TestCase
 
       context "with a banned artist" do
         setup do
-          @artist = FactoryGirl.create(:artist)
+          CurrentUser.scoped(FactoryGirl.create(:admin_user)) do
-          @artist.ban!
+            @artist = FactoryGirl.create(:artist)
+            @artist.ban!
+          end
           @post = FactoryGirl.create(:post, :tag_string => @artist.name)
         end