t1405: mark test for 'git pack-refs' as REFFILES

t1405: mark test for 'git pack-refs' as REFFILES

The tests verifies that "pack-refs" causes loose refs to be packed. As both
loose and packed refs are concepts specific to the files backend, mark the test
as REFFILES.

Check the outcome of the pack-refs operation. This was apparently forgotten in
the commit introducing this test: 16feb99d (Mar 26 2017, "t1405: some basic
tests on main ref store").

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>