Test::Group is deeply in bed with Test::Builder and is broken as of alpha 2. The primary problem as of alpha 2 is it makes heavy use of subtests and those are broken. The problem may go away when subtests are fixed.
It's working pretty good now that subtests are fixed, but tests fail because of the TAP version header.
Deal with that later.
The tests do a lot of equality checks on the TAP and will be difficult to adapt to Test::Builder::Tester. On the up side, nothing much depends on it.