Remove the fail case on IPv6 in testLockdownVpn

In testLegacyLockdownVpn, remove the fail check on IPv6 networks
and add coverage in VpnTest instead as this interaction relies
on the Vpn implementation, not ConnectivityService.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Ib24809ece2332c4c3d3e08c168e02ad859242eac
2 files changed