[RFC] blackbox tests

[Jamie Wilkinson]

This one time, at band camp, Robert Collins wrote:
>This will reduce knock-on effects in the test suite when one command
>changes - only the tests for that command should be effected. It will
>also put pressure on the library API to provide the same functionality
>the CLI does, to avoid DRY, which will have the nice benefit of making
>it easier for plugin authors and GUI writers to do what the UI does
>without duplication.
>
>Seeking a +1 on this as a 'preferred approach', something for us to look
>for in reviews, and to eventually change the current tests to be like.

+1 and it should go into the wiki or somewhere visible as a guide to
writing BB tests.