EthernetTetheringTest: test RFC 7050 prefix64 discovery

Reply ipv4only.arpa AAAA query with Pref64::WKA for applying test
prefix64 to LinkProperties of test network.
See Nat464Xlat#setNat64PrefixFromDns.

Currently, the test prefix64 is applied directly in test network
initialization. In order to increase the NAT64 test coverage,
the integration test starts to reply prefix64 discovery DNS query
for testing the RFC 7050 prefix discovery.

These clat related tests are still expected to pass since this
prefix64 source change:
- testTetherClatIcmp
- testTetherClatUdp
- testTetherClatTcp

Bug: 259888307
Test: atest EthernetTetheringTest
Change-Id: Ic94ce997dd0b1d718a1074850c01ea90298cc1c3