selftests: netfilter: skip tests on early errors

selftests: netfilter: skip tests on early errors

br_netfilter: If we can't add the needed initial nftables ruleset skip the
test, kernel doesn't support a required feature.

rpath: run a subset of the tests if possible, but make sure we return
the skip return value so they are marked appropriately by the kselftest
framework.

nft_audit.sh: provide version information when skipping, this should
help catching kernel problem (feature not available in kernel) vs.
userspace issue (parser doesn't support keyword).

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240423130604.7013-7-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>