selftests/bpf: Fix bpf_nf selftest failure

For systems with missing iptables-legacy tool this selftest fails.

Add check to find if iptables-legacy tool is available and skip the
test if the tool is missing.

Fixes: de9c8d84 ("selftests/bpf: S/iptables/iptables-legacy/ in the bpf_nf and xdp_synproxy test")
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250409095633.33653-1-skb99@linux.ibm.com