tests: Guard against testing without scapy

[jlduran]

Summary

Add a guard that checks if a system has scapy installed, to avoid
reporting false positives in the testing framework.

This guard is added in the utilities subroutine instead of the
traditional:

atf_set "require.progs" "python3" "scapy"

in the header of each test using pft_ping.py.

The trade-off is that the test takes a little longer to be skipped.

Reported by: Jenkins

[jlduran]

Current false positives (on architechtures other than amd64 or i386 [^1]):

• sys.netpfil.pf.fragmentation_pass.dummynet_fragmented
• sys.netpfil.pf.src_track.max_src_states_rule
• sys.netpfil.pf.src_track.max_src_conn_rule
• sys.netpfil.pf.max_states.max_states
• sys.netpfil.pf.route_to.ifbound_reply_to_rdr_dummynet
• sys.netpfil.pf.route_to.ifbound_reply_to
• sys.netpfil.pf.route_to.ifbound_reply_to_v6
• sys.netpfil.pf.fragmentation_pass.no_df
• sys.netpfil.pf.fragmentation_pass.reassemble_slowpath

If tests strongly prefers to add atf_set "require.progs" "python3" "scapy", I'll change the revision. However, this review aims to avoid having this issue recurrently.

[^1]: See https://github.com/freebsd/freebsd-ci/blob/476c8808803a47344a47adfe9573345cceb14fce/scripts/build/build-test_image-head.sh#L63-L64 for more info.

[kp]

I think there's a preference for expressing this in the head of the test, not the body. Igor is thinking about ways to teach kyua to satisfy these requirements automatically, which it won't be able to do if the test is done at run-time. This approach also leaves us with tests that execute part of the test and then potentially skip the rest of it.

Also, as the scapy package depends on the python one I don't see the need to also depend on python3.