[MERGE] Convert tests.blackbox.test_add to use internals where appropriate

[John Arbash Meinel]

John Arbash Meinel has voted approve.
Status is now: Semi-approved
Comment:
you know, you can send bigger requests than this :)

Mechanical changes are a bit easier to review than code changes, so you 
can send more at a time.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46B21086.6080505%40warwick.ac.uk%3E