use escaped "Can't" constant at IssuesHelperTest

git-svn-id: http://svn.redmine.org/redmine/trunk@12842 e93f8b46-1217-0410-a6f0-8f06a7374b81
Rails4: replace hard-coded html with class at ApplicationHelperTest#test_wiki_links_within_wiki_page_context

See r12784 comment.

git-svn-id: http://svn.redmine.org/redmine/trunk@12843 e93f8b46-1217-0410-a6f0-8f06a7374b81