t6030: use modern test_* helpers

t6030: use modern test_* helpers

We can get rid of a lot of hand-rolled error messages by
using test_must_fail and test_expect_code. The existing code
was careful to use "|| return 1" when breaking the
&&-chain, but it did fool --chain-lint; the new code is more
idiomatic.

We also add some uses of test_when_finished, which is less
cryptic and more robust than putting code at the end of a
test. In two cases we run "git bisect reset" from a
subshell, which is a problem for test_when_finished (it
would not run). However, in both of these cases, we are
performing the tests in one-off sub-repos, so we do not need
to clean up at all (and in fact it is nicer not to if the
user wants to inspect the trash directory after a failure).

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