vmtests: `pkg.sensors.tracing:TestKprobeRateLimit` and `NoRateLimit` are failing silently because of `panic` in tests

[Mahé]

I realized that the test was failing while writing the v1.0 backport #2584. Only to realize later that it was also failing on main and v1.1, while the vmtests recap being green (for example see, a v1.1 run or a main run). So it was failing everywhere because nc.openbsd was missing.

So it seemed that when tests panic, they are failing according to vmtests but not counted toward the total. The only remaining question is: what is the difference between v1.0 and v1.1/main?

So I see a few things:

• Fix the tests to use Fatal or FailNow
  #2685
• Add nc.openbsd to the base image used by vmtests
  cilium/little-vm-helper-images#582

Also the test is really slow on success (40s+) because it tries to check for missing events (so it's retrying multiple time, it should be optimized not to make the test suite too slow.