selftests/bpf: Better verification in test_tcpbpf

Reduce amount of copy/paste for debug info when result is verified in
the test and keep that info together with values being checked so that
they won't get out of sync.

It also improves debug experience: instead of checking manually what
doesn't match in debug output for all fields, only unexpected field is
printed.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>