Let DW::FormErrors be called from a test

Make sure we include DW::Request explicitly instead of relying on the
environment to load it for us.
Remove duplicate plan lines
Add trailing whitespace to expected output

(Significant for Markdown processing)
Check response content type, not request content type
Turn on the track_thread cap to test that tracking threads works
Add admin_post and chal to the "posted" form
Merge pull request #782 from afuna/fix-miscellaneous-tests

Fix miscellaneous tests