Seeing nobody else has gotten around to opening a bug for this, I will. Presently there's both USE="test" and FEATURES="test". FEATURES="test" enables src_test() in ebuilds which sometimes requires other packages which aren't required in any other case. USE="test" is currently used to bring in these other packages. There's also some cases where further SRC_URI tarballs are required for the testing phase, which is also presently handled with USE="test". The two need settings need to be combined into a single parameter.
uhh, what would you call Bug 69021 then ?
This bug's duplicate ;) *** This bug has been marked as a duplicate of 69021 ***