tests: Require python3 when using Scapy

python3 is a symbolic link that points to the current python 3.X
version.

It is possible for a system to have python (python 3.X) without the
python3 (symlink) package.

Test scripts that use Scapy are invoked using python3, so add it as a
required program.

Reviewed by: ngie, asomers
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D49007