system-tests: Run conntrack tests with userspace.

system-tests: Run conntrack tests with userspace.

The userspace connection tracker doesn't support ALGs, frag reassembly
or NAT yet, so skip those tests.

Also, connection tracking state input from a local port is not possible
in userspace.

Finally, the userspace datapath checks for the IPv4 header checksum, so
fix those in the hardcoded packets.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>