netinet6 tests: Add Scapy as a required program

The ra.py script requires Scapy. Invoke it using the python3 symlink.
While technically, at the moment, python points to python3, and python3
points to the current version of python, it is recommended to use
python3. No functional change intended.

Add the Scapy requirement in the header of each test that makes use of
it.

Reviewed by: kp
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49005