ping: fix listing test cases when scapy is not installed

The ATF-python test program was attempting to list test cases that
require scapy. But it attempted to import the scapy module before the
test cases had been listed, resulting in an ImportError that kyua
interpreted as a test program crash.

Fix this behavior by handling that ImportError well enough to list test
cases, but not run them. If scapy isn't present, Kyua will refuse to
run the test cases. But it needs to be able to list them in order to
know to skip them.

Sponsored by: ConnectWise
MFC after: 2 weeks
Reviewed by: maxim
Differential Revision: https://reviews.freebsd.org/D56765