tests: Fix `bgp_srv6l3vpn_to_bgp_vrf3` topotest

tests: Fix `bgp_srv6l3vpn_to_bgp_vrf3` topotest

The `bgp_srv6l3vpn_to_bgp_vrf3` topotest tests the SRv6 L3VPN
functionality. It applies the appropriate configuration in `bgpd` and
`zebra`, and then checks that the RIB is updated correctly.

The topotest expects to find the AS-Path in the RIB, which is only
present if the `bgp send-extra-data zebra` option is enabled in the
`bgpd` configuration.

The `bgp send-extra-data zebra` option has been accidentally commented
out in commit https://github.com/FRRouting/frr/commit/2007e2dbd0d5c42d9fe6cbe92b34be10654834ef.

This commit fixes the `bgp_srv6l3vpn_to_bgp_vrf3` topotest by re-adding
the missing `bgp send-extra-data zebra` option.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>