Fix broken links in troubleshooting section

- The link to the non-existent `cascalog.testing` namespace was changed
  to midje.cascalog and now points to the midje.cascalog directory in
  the cascalog repo.

- The link to Cascalog's own tests now points to the
  cascalog-core/test/cascalog directory within the cascalog repo.

- The link to Sam Ritchie's Cascalog Testing 2.0 post was corrected. Was
  previously returning 404.