test-lib: turn on ASan abort_on_error by default

test-lib: turn on ASan abort_on_error by default

By default, ASan will exit with code 1 when it sees an
error. This means we'll notice a problem when we expected
git to succeed, but not in a test_must_fail block.

Let's ask it to actually raise SIGABRT instead. That will
give us a signal death that test_must_fail will notice. As a
bonus, it may also leave a coredump, which can be handy for
digging into a failure.

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