selftest/bpf: Add missed ip6ip6 test back

In comment 173ca26e ("samples/bpf: add comprehensive ipip, ipip6,
ip6ip6 test") we added ip6ip6 test for bpf tunnel testing. But in commit
933a741e ("selftests/bpf: bpf tunnel test.") when we moved it to
the current folder, we didn't add it.

This patch add the ip6ip6 test back to bpf tunnel test. Update the ipip6's
topology for both IPv4 and IPv6 testing. Since iperf test is removed as
currect framework simplified it in purpose, I also removed unused tcp
checkings in test_tunnel_kern.c.

Fixes: 933a741e

 ("selftests/bpf: bpf tunnel test.")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201110015013.1570716-2-liuhangbin@gmail.com