topotests: raise an error if pinging from vrf is not possible

topotests: raise an error if pinging from vrf is not possible

Because of the issue described in the above link, pinging from vrf with
the command "ip vrf exec <vrf> ping -I <src> <addr>" may fail.

> root@topo:~# ip vrf exec vrf1 ping -c1 -I 192.168.2.1 192.168.1.1
> bind: Cannot assign requested address

Raise an error if pinging its own IP from a VRF fails. This test should
always work unless in the condition of this issue.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=203483
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>