t: use test_must_fail instead of hand-rolled blocks

t: use test_must_fail instead of hand-rolled blocks

These test scripts likely predate test_must_fail, and can be
made simpler by using it (in addition to making them pass
--chain-lint).

The case in t6036 loses some verbosity in the failure case,
but it is so tied to a specific failure mode that it is not
worth keeping around (and the outcome of the test is not
affected at all).

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