selftests: netfilter: nft_fib.sh: check lo packets bypass fib lookup

selftests: netfilter: nft_fib.sh: check lo packets bypass fib lookup

With reverted fix:
PASS: fib expression did not cause unwanted packet drops
[   37.285169] ns1-KK76Kt nft_rpfilter: IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.0.0.1 DST=127.0.0.1 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=32287 DF PROTO=ICMP TYPE=8 CODE=0 ID=1818 SEQ=1
FAIL: rpfilter did drop packets
FAIL: ns1-KK76Kt cannot reach 127.0.0.1, ret 0

Check for this.

Link: https://lore.kernel.org/netfilter/20250422114352.GA2092@breakpoint.cc/
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>