better diagnostics in tests: capture log_debug messages too; dump all messages on error

...to help decipher failure cases:
http://cpantesters.org/cpan/report/87a37470-6bf4-1014-953e-8bc38887c6f4
Dist-Zilla-Plugin-PromptIfStale-0.010-TRIAL

          - more diagnostics printed in failing tests