selftest: fix acl_xattr test: sn-devel unreliable gid

selftest: fix acl_xattr test: sn-devel unreliable gid

The "nt_affects_chgrp" kept failing in a full autobuild on sn-devel
because the actual gid of the created file as returned by smbclient -c
getfacl was reliably the unix gid of my account. It should have been the
mapped domusers group for the primary users "Domain Users"
group. Running the test individually or even the full set of
"samba3.blackbox" tests didn't trigger the error.

Looks like an issue with vfs_fake_acls and vfs_xattr_tdb, but I wasn't
able to track it down. As the test only really want to ensure that
smbcacls -G set the gid to the requested value, just remove the check
for the actual initial gid.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=7933

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>