tests: validate generated netlink instructions

tests: validate generated netlink instructions

compare netlink instructions generated by given nft command line
with recorded version.

Example: udp dport 80 accept in ip family should look like

ip test-ip4 input
  [ payload load 1b @ network header + 9 => reg 1 ]
  [ cmp eq reg 1 0x00000011 ]
  [ payload load 2b @ transport header + 2 => reg 1 ]
  [ cmp eq reg 1 0x00005000 ]
  [ immediate reg 0 accept ]

This is stored in udp.t.payload.ip

Other suffixes:
.payload.ip6
.payload.inet
.payload ('any')

The test script first looks for 'testname.t.payload.$family', if that
doesn't exist 'testname.t.payload' is used.

This allows for family independent test (e.g. meta), where we don't
expect/have any family specific expressions.

Signed-off-by: Florian Westphal <fw@strlen.de>