Add kernel test suite

[javierhonduco]

Given how many (bleeding edge) BPF features bpfilter uses it would be neat to have a kernel testing suite to ensure that the various BPF programs can at least load without issues in some supported kernels.

This is how I do this for one of my projects. This is very useful in general but as lightswitch does runtime feature probing to decide what helpers and other BPF features to use, it's the only way to ensure there are no regressions.

Related issue: #567

[qdeslandes]

I think there are two different issues here:

• Add a kernel test suite to run bpfilter test against different kernel and ensure we handle unsupported features properly. This would mostly be about bpfilter's behaviour, because, as you mentioned in a different issue, distro kernels often backport features, so we can't rely on the kernel version
• Implement runtime probing in bpfilter to error out if a specific feature can't be used because the current kernel doesn't support it