Improve process group killing

What does this MR do?

Improves KillProcessGroup function to fix a bugs that may occur on Unix based environments with shell executor.

Why was this MR needed?

Please read #1662 (closed) for details

Are there points in the code the reviewer needs to double check?

The construction of KillProcessGroup function and process_group_unix_test.go file.

Test artifacts to download

https://s3.amazonaws.com/gitlab-ci-multi-runner-downloads/fix/process-killing-bug/index.html

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Fixes #1662 (closed)