:bug: ensure envtest stops the whole process group

A previous pull request attempted to prevent envtest from leaking
orphaned etcd / kube-apiserver etc. processes by adding a SysProcAttr of
`Setpgid: true`. However, it made a few mistakes:

- It incorrectly described the change as causing the process to "inherit
  the parent's process group id", which is the opposite of what it does.
  In fact it causes a brand new process group to be created for the child
  process, which is the actual behaviour we want, so correct the comment.
- Whilst it added a new process group, it didn't update the signalling
  to send SIGTERM (and eventually SIGKILL) to the process group (by
  using a `-` prefix) and was still just sending the signal to the process
  itself.
- It didn't add a unittest to assert on the desired behaviour before and
  after the changes.

This PR should hopefully fix all of those problems and result in fewer
process leaks.

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>