Use #assert_select instead of #assert_tag.

git-svn-id: http://svn.redmine.org/redmine/trunk@12647 e93f8b46-1217-0410-a6f0-8f06a7374b81
Adds rel=stylesheet condition for stylesheets assertions.

git-svn-id: http://svn.redmine.org/redmine/trunk@12648 e93f8b46-1217-0410-a6f0-8f06a7374b81