Battery: fix plugin initialization, again!

AngelPS1::System->fetch_battery returns a sub, and
AngelPS1::System->can('fetch_battery') returns a sub that returns that
sub.
This bug was introduced because t/53-Battery.t had a fake
AngelPS1::System that was not correctly implemented.
This patch restores the battery status that had disappeared.
t/lib: add AngelPS1::Shell::Raw, useful for tests
tests: use AngelPS1::Shell::Raw (from t::lib)