selftests: mptcp: fix ipv6 routing setup

MPJ ipv6 selftests currently lack per link route to the server
net. Additionally, ipv6 subflows endpoints are created without any
interface specified. The end-result is that in ipv6 self-tests
subflows are created all on the same link, leading to expected delays
and sporadic self-tests failures.

Fix the issue by adding the missing setup bits.

Fixes: 523514ed ("selftests: mptcp: add ADD_ADDR IPv6 test cases")
Reported-and-tested-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>