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: #6087
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Johan Bergström <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>
Reviewed-By: Claudio Rodriguez <[email protected]>

Author: santigimeno