selftests: forwarding: Add MPLS L2VPN test

Connect hosts H1 and H2 using two intermediate encapsulation routers
(LER1 and LER2). These routers encapsulate traffic from the hosts,
including the original Ethernet header, into MPLS.

Use ping to test reachability between H1 and H2.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/625f5c1aafa3a8085f8d3e082d680a82e16ffbaa.1606918980.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>