test_bpf: Add a couple of tests for BPF_JSGE.

Some JITs can optimize comparisons with zero.  Add a couple of
BPF_JSGE tests against immediate zero.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>