test-tool: help verifying BUG() code paths

test-tool: help verifying BUG() code paths

When we call BUG(), we signal via SIGABRT that something bad happened,
dumping cores if so configured. In some setups these coredumps are
redirected to some central place such as /proc/sys/kernel/core_pattern,
which is a good thing.

However, when we try to verify in our test suite that bugs are caught in
certain code paths, we do *not* want to clutter such a central place
with unnecessary coredumps.

So let's special-case the test helpers (which we use to verify such code
paths) so that the BUG() calls will *not* call abort() but exit with a
special-purpose exit code instead.

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>