test-meta.sh: use user's uid/gid, not 0

Cygwin does not fully support uid and gid of 0. Instead, use the
current user's ids to avoid portability problems. Since this test is
not run for ordinary users, the result is the same.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
Avoid uid/gid 0 metadata tests when ids don't exist

Cygwin may not have a 0 uid/gid, so skip the relevant tests whenever
it doesn't.

We have to explicitly exclude the gid 0 from other_group, because
/etc/group may define a group named root with the same id as
Administrators. We cannot use "root" instead of 0, because root may
not be defined.

Cygwin users: If you want to define the root group, add this line
at the begining of /etc/group:
  root:S-1-5-32-544:0:

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>