reflog tests: test for the "points nowhere" warning

reflog tests: test for the "points nowhere" warning

The "git reflog expire" command when given an unknown reference has
since 4264dc15e1 ("git reflog expire", 2006-12-19) when this command
was implemented emit an error, but this has never been tested for.

Let's test for it, also under gc.reflogExpire{Unreachable,}=never in
case a future change is tempted to take shortcuts in the presence of
such config.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>