BPF CI update: veristat-scx job

[Ihor Solodrai <ihor.solodrai@linux.dev>]

Hi everyone.

In effort to improve testing of both BPF subsystem and sched-ext
project [1], a new testing workflow has been added to BPF CI:
veristat-scx.

veristat-kernel and veristat-meta jobs have been running for a while
now, and veristat-scx is basically adding more test cases using
sched-ext BPF programs as input.

For those who aren't aware, veristat [2] is a command line tool that
can be used to load BPF object files into the kernel and check
verification results, among other things. It's source code is at
./tools/testing/selftests/bpf/veristat.c

On BPF CI, veristat-${target} job takes BPF object files (determined
by ${target}) as input, and runs veristat on them against the kernel
under testing. In addition to checking whether BPF program is accepted
by the verifier, the CI job also collects verifier performance stats,
and compares them to the baseline *failing* if a significant
regression is detected.

See an example of successful job run here:
https://github.com/kernel-patches/bpf/actions/runs/15543439297/job/43761685117

[1] https://github.com/sched-ext/scx
[2] https://github.com/libbpf/veristat