tests: fixup comment vote, post controller tests.

Use git clean to delete stale files from an earlier checkout. These
files interfered with various things (specifically, a comment vote test
failed because there was an old comment_votes/create.json.erb template
hanging around that changed the behavior of the POST /comment_votes endpoint).

config/docker/danbooru-ci

@@ -25,5 +25,6 @@ RUN sed -i'' -e 's/run -it/run -i/' /usr/local/bin/circleci
 
 USER danbooru
 RUN git reset --hard master
+RUN git clean --force
 RUN gem install bundler --version $BUNDLER_VERSION
 RUN bundle install > /dev/null