Tests that use TestMain might never call m.Run(), and simply return
from TestMain. In that case, the iOS test harness never sees the
PASS from the testing framework and assumes the test failed.

Allow an exit with exit code 0 to also mean test success, thereby
fixing the objdump test on iOS.

Change-Id: I1fe9077b05931aa0905e41b88945cd153c5b35b6
Reviewed-on: https://go-review.googlesource.com/36065Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>