os/user: don't skip TestLookupGroup if supported

CL 37664 implemented this functionality, yet the tests were skipped.

Introduce and use additional variable groupListImplemented to
distinguish between these cases and enable TestLookupGroup for
supported configurations (which looks like all but plan9).

Change-Id: Iabaa7f08b4551dc67e67bdb6e715f15bb20d6218
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/330751


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>