misc/py-shell-gpt: don't install test files

py-shell-gpt and py-notus-scanner both try to install test files to a
generic location (/usr/local/lib/python3.9/site-packages/tests).
py-shell-gtp does not require test files for normal operations, so
avoid collision by not installing test files.

PR: 270542
Reported by: John Hein <jcfyecrayz@liamekaens.com>