test: make test-process-argv-0 robust

Remove use of STDERR to avoid test flakiness on CentOS.

Fixes: https://github.com/nodejs/node/issues/2477

Additional benefit: This makes the test a fair bit faster.

Before:

real 0m0.909s user 0m0.216s sys 0m0.083s

After:

real 0m0.223s user 0m0.193s sys 0m0.035s