add missing fixture to ApplicationHelperTest

git-svn-id: http://svn.redmine.org/redmine/trunk@13172 e93f8b46-1217-0410-a6f0-8f06a7374b81
use with_settings at AttachmentsControllerTest#test_show_text_file_should_send_if_too_big

git-svn-id: http://svn.redmine.org/redmine/trunk@13173 e93f8b46-1217-0410-a6f0-8f06a7374b81
add test of attachment download etag

git-svn-id: http://svn.redmine.org/redmine/trunk@13174 e93f8b46-1217-0410-a6f0-8f06a7374b81