Add a test helper plugin

This currently adds Code Coverage abilities, but this may be expanded later.
See #269.

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2151 f69abe3c-2ee2-4550-9a6d-17bd458115be
Check that the handler key exists before using it

This previously caused a warning, which I completely missed.

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2152 f69abe3c-2ee2-4550-9a6d-17bd458115be