tests: use dynamic python path

Stop hardcoding the Python path to /usr/bin/python3. Not all systems
have Python installed to /usr/bin, and some users might have installed
Python to a custom location.

Instead, use /usr/bin/env, which performs a $PATH lookup to find the
Python executable.

Signed-off-by: Issam E. Maghni <issam.e.maghni@mailbox.org>