Add test assertion to check cell attributes


git-svn-id: svn+ssh://svnrepo/projects/tachyon/trunk@7572 561c3618-0fe6-0310-b5d6-fc65c73fee30
When asserting a string and it doesn't match, dump the framebuffer to aid
debugging


git-svn-id: svn+ssh://svnrepo/projects/tachyon/trunk@7573 561c3618-0fe6-0310-b5d6-fc65c73fee30