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