QuickFix : dependency cycle in csrf_defender tests.

Those tests require A::P::Web::HTTPSession. Bu this distribution needs
Amon2.

For now, just skip the tests if the plugin is not installed. This is a
quick fix until I can find a clean solution to still have the
CRSFDefender features properly tested.
Merge pull request #84 from GeJ/PR_504_Deps

QuickFix : dependency cycle in csrf_defender tests.