Replace changeset_comments fixtures with a factory.

There's little point in testing ChangesetComment.count so I've removed that
test.

test/factories/changeset_comments.rb

@@ -0,0 +1,12 @@
+FactoryGirl.define do
+  factory :changeset_comment do
+    sequence(:body) { |n| "Changeset comment #{n}" }
+    visible true
+
+    # FIXME: needs changeset factory
+    changeset_id 3
+
+    # FIXME: needs user factory
+    author_id 1
+  end
+end