Guard Process.kill pid 1 spec in case user owns it

In some environments, the user running the spec might own pid `1` (even without being root).  It's also possible this spec could be run from _as_ pid `1`.  This is the case with Docker, in which the command you pass to `docker run` is run as pid 1.

```
jemc@localhost~/1/docker> sudo docker run -t -i rubinius ps
  PID TTY          TIME CMD
    1 ?        00:00:00 ps
```

In this situation, MRI will allow killing of pid 1 without exception (and so will `kill -9 1`).

The spec needs to guard this test and skip it when the current user owns pid 1.
Is there a way we could test this without relying on shell commands? If not, is it worth checking for in the first place? Perhaps we shouldn't use PID 1 for tests in the first place.