selftests: netfilter: nft_audit.sh: add more skip checks

This testcase doesn't work if auditd is running, audit_logread will not
receive any data in that case.

Add a nftables feature test for the reset keyword and skip this test
if that fails.

While at it, do a few minor shellcheck cleanups.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-12-fw@strlen.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>