test-lib: add a --invert-exit-code switch

test-lib: add a --invert-exit-code switch

Add the ability to have those tests that fail return 0, and those
tests that succeed return 1. This is useful e.g. to run "--stress"
tests on tests that fail 99% of the time on some setup, i.e. to smoke
out the flaky run which yielded success.

In a subsequent commit a new SANITIZE=leak mode will make use of this.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>