selftests: netfilter: Extend nft_audit.sh

Add tests for sets and elements and deletion of all kinds. Also
reorder rule reset tests: By moving the bulk rule add command up, the
two 'reset rules' tests become identical.

While at it, fix for a failing bulk rule add test's error status getting
lost due to its use in a pipe. Avoid this by using a temporary file.

Headings in diff output for failing tests contain no useful data, strip
them.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>