Add the "sys" group for tests.

It's useful to have a group that shares a numeric id with Linux and
macOS. "root" doesn't count because group 0 is actually called "wheel"
on the BSDs, and macOS has "daemon" and "kmem" as its groups 1 and 2.

This lets us have toybox tar tests that have the same results on all
supported OSes without special handling.

Test: treehugger
Change-Id: I4704d6f9ada84f3065831a099b34d1c20c4c6b03