t: mark several tests that assume the files backend with REFFILES

t: mark several tests that assume the files backend with REFFILES

Add the REFFILES prerequisite to several tests that assume we're using
the files backend. There are various reasons why we cannot easily
convert those tests to be backend-independent, where the most common
one is that we have no way to write corrupt references into the refdb
via our tooling. We may at a later point in time grow the tooling to
make this possible, but for now we just mark these tests as requiring
the files backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>