test: simplify return value of test_run_

test: simplify return value of test_run_

As v0.99.5~24^2~4 (Trapping exit in tests, using return for errors,
2005-08-10) explains, callers to test_run_ (such as test_expect_code)
used to check the result from eval and the return value separately so
tests that fail early could be distinguished from tests that completed
normally with successful (nonzero) status.  Eventually tests that
succeed with nonzero status were phased out (see v1.7.4-rc0~65^2~19,
2010-10-03 and especially v1.5.5-rc0~271, 2008-02-01) but the weird
two-return-value calling convention lives on.

Let's get rid of it.  The new rule: test_run_ succeeds (returns 0)
if and only if the test succeeded.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>