modify test so it passes even in the presence of warnings from external modules

e.g. perl 5.21.6 + aliased.pm:
http://www.cpantesters.org/cpan/report/0d3cdd02-6d88-11e4-8a4f-4a6310862755
Dist-Zilla-Plugin-Test-EOL-0.16

        - allow tests to pass even while upstream modules have warnings under
          perl 5.21.6