EVPN IRB test case with anycast gateway

[jbemmel]

There are currently no integration tests that combine EVPN/VXLAN with anycast gateways

On EOS it is suggested there are 2 different anycast configurations (for IPv4):

1. ip virtual-router address (currently used)
2. ip address virtual (IPv4 only, only flavor supported in combination with VXLAN)

The latter assumes no unicast IPv4 address on the interface