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
Merged-In: I04de6e3c7b1400a5a57daa1bf855330c20516db7
Change-Id: I9c6ea40e4a73d53ff460f8e2d34a6c2a605d34b5
(cherry picked from commit b5b3ce5a)