topotests: bgp_srv6l3vpn_to_bgp_vrf locator update test case

topotests: bgp_srv6l3vpn_to_bgp_vrf locator update test case

If the SRv6 locator is deleted in zebra, zclient(bgpd)
which allocates SIDs from the locator will update the
RIBs which use those SIDs and make them invalid.
This will cause the VPNv6 route to be withdrawn and
the VPN to stop.

If the SRv6 locator is added again, zclient(bgpd) will
allocate the SIDs from the locator again, and VPNv6
will be re-established.

This commit add a test case to confirm this.

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>