t1301: loosen test for forced modes

t1301: loosen test for forced modes

One of the aspects of the test checked explicitly for the
g+s bit to be set on created directories. However, this is
only the means to an end (the "end" being having the correct
group set). And in fact, on systems where
DIR_HAS_BSD_GROUP_SEMANTICS is set, we do not even need to
use this "means" at all, causing the test to fail.

This patch removes that part of the test. In an ideal world
it would be replaced by a test to check that the group was
properly assigned, but that is difficult to automate because
it requires the user running the test suite be a member of
multiple groups.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>