selftests: netfilter: Fix nft_audit.sh for newer nft binaries

As a side-effect of nftables' commit dbff26bfba833 ("cache: consolidate
reset command"), audit logs changed when more objects were reset than
fit into a single netlink message.

Since the objects' distribution in netlink messages is not relevant,
implement a summarizing function which combines repeated audit logs into
a single one with summed up 'entries=' value.

Fixes: 203bb9d3 ("selftests: netfilter: Extend nft_audit.sh")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>