Add tests for existing functionality
Correctly log pass/fails for some negative checks

For 'should not' checks, where the result did not appear at all, we were
correctly recording that it passed, and adding it to the score, but we logged a
message that said it failed.

This is because the logging was disconnected from the "success" to some extent.
Make expectation-setting clearer

"foo" was a stupid name, but it was also not clear that there was an
expectation being set there.
Merge pull request #126 from alphagov/treat_non_appearance_as_pass_for_should_not_checks

Treat non appearance as pass for 'should not' checks