Fix the flaky test MaxServerPrune_Binder

This change fixes resolv_integration_test getting stuck.
It happens when DNSResponder has terminated while DnsTlsFrontend
is still waiting for the response from the DNSResponder, which causes
DnsTlsFrontend loop thread can't be terminated.

Bug: 119694273
Test: system/netd/tests/runtests.sh passed
Change-Id: I04de6e3c7b1400a5a57daa1bf855330c20516db7