From discussion on IRC, it seems, like there are different options, so i would like to clarify this
The test USE flag is (i am only talking about portage now, since i am most familar with it) an
internal flag, which is added by portage for every ebuild and enabled/disabled based on
FEATURES=test or not. It is not managed via USE= line in make.conf or package.use settings. So in
handling and setting, it is pretty much the same as for the arch USE flags, which are also only
internally used and added, but never exposed to the users.
Now i see the opinion, that it should always be added to IUSE, when it is referenced in the ebuild.
This adds it to the visible USE flags in emerge output (imho pointless, since it is no option, which
can be enabled/disabled like the other USE flags), but otherwise does change nothing.
Because of this, i would like to discourage the addition of test USE flag to IUSE, since it is just
an internal USE flag, which should not be exposed to the user.
Gentoo Linux Developer