selftests: vrf_route_leaking: add local test

The goal is to check that the source address selected by the kernel is
routable when a leaking route is used. ICMP, TCP and UDP connections are
tested.
The symmetric topology is enough for this test.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240710081521.3809742-5-nicolas.dichtel@6wind.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>