Add tests for interesting CPAN modules

[schwern]

We're messing around with things which could affect compatibility, particularly undocumented XS stuff. It will be useful to catch problems early by building and testing various "interesting" CPAN modules.

"Interesting" can mean...

• Important to users (used often, or a common dependency)
  • Moose
  • Test::More
  • DBI
• Exercises parts of Perl that we might break (usually XS)
  • XML-LibXML
  • Text-CSV_XS

We'll find more examples as we break things. 😁

• Pick a few distributions for testing.
• Put them in t/cpan/
  • Simple builds should go into one file.
  • Complicated builds (like DBD::mysql) should have their own file.
• Only run them with make test_cpan.
• Add make test_cpan to the .travis.yml