Added debug.log to log echos inside tests.

- Uses verbosefile, so will also show messages that other plugins will report.
- An alternative was :redir to trap all echos, but that may interfere with plugins
  that need to capture ex command output. Since :redir can't be nested, we could
  potentially alter plugin functionality. Avoid.
- Another alternative was to introduce a logger object with .info, .fail etc.
  This would need modification to the dsl, and custom code to handle variadic args
  that echo already handles quite well.
- verbosefile is our best bet for now.