We want to migrate C++ test jobs to kokoro macservice worker, but currently the dualstack_socket_test is failing there.
See b/201064791

The issue is that macservice workers are using DNS64 name servers, and when resolving loopback4.unittest.grpc.io, it gives a correct, but unexpected result for loopback4.unittest.grpc.io has IPv6 address 64:ff9b::7f00:1 (it's a IPv4 address wrapped in IPv6 - which is something that's currently not supported by dualstack_socket_test)

$host loopback4.unittest.grpc.io 2001:4860:4860::6464
Using domain server:
Name: 2001:4860:4860::6464
Address: 2001:4860:4860::6464#53
Aliases:

loopback4.unittest.grpc.io has address 127.0.0.1
loopback4.unittest.grpc.io has IPv6 address 64:ff9b::7f00:1

The workaround is to detect whether DNS64 servers are used (they will be on kokoro macservice workers) and skip the "*.unittest.grpc.io" tests if so.