selftests: bpf: add trivial JSET tests

We seem to have no JSET instruction test, and LLVM does not
generate it at all, so let's add a simple hand-coded test
to make sure JIT implementations are correct.

v2:
 - extend test_verifier to handle multiple inputs and
   add the sample there (Daniel)
 - add a sign extension case

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>