test: put test_prctl and test_clearenv under test.rs

[SteveLauC]

What does this PR do

Move test_prctl and test_clearenv to test.rs

I actually have no idea why they are put under their own sections, but it seems to be ok to let them live with other tests.

No actual code changes in test/sys/test_prctl.rs, I just removed

#[cfg(target_os = "linux")]
#[cfg(feature = "process")]
mod test_prctl {
}

and shifted the code

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

The prctl stuff is in its own test program because it changes process-wide settings that might affect other tests.
#1550 (comment) . clearenv also affects the process-wide environment.

[SteveLauC]

The prctl stuff is in its own test program because it changes process-wide settings that might affect other tests. #1550 (comment) . clearenv also affects the process-wide environment.

Makes sense, they should be put in their own processes, close this PR