gctl_test.t: minor tweaks

gctl_test.t: minor tweaks

- Declare $count with the `my` scope operator to permit `use strict`.
- Add `use strict`.
- Use `use warnings` instead of using `-w` in the shebang.
- Don't unlink $cmd when done (prevents unnecessary rebuilding).
- Improve the error message when running with insufficient permissions, e.g.,
  non-root.

MFC after:	5 weeks
Sponsored by:	Dell EMC Isilon