[test] fix test_printable_value_bytes with current pygments
An empty span was added in
https://bitbucket.org/birkenfeld/pygments-main/commits/164574c13533195a555181a2b9c685fea2470403
(grafted from 74b04a88d28a)
cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)