test: move some test from sequential to parallel

[santigimeno]

Checklist

• tests and code linting passes
• the commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

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.

I have run the tests on Debian Jessie 64 and SmartOS and pass on both.