test: move some test from sequential to parallel

The only test with modifications is `test-stdin-child-proc` that was
passing when it should not because the exit code of the child process
was not being checked.

PR-URL: https://github.com/nodejs/node/pull/6087


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>