t: use sane_unset() rather than 'unset' with broken &&-chain
These tests intentionally break the &&-chain after using 'unset' since
they don't know if 'unset' will succeed or fail and don't want a local
'unset' failure to fail the test overall. We can do better by using
sane_unset(), which can be linked into the &&-chain as usual.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>