Clear the testing resolver configuration after the tests finish

This change removes the resolver caches used for the testing networks
in order not to waste memory.

Bug: 130202604
Test: as follows
    - runtests.sh resolv_integration_test
    - The testing networks are not shown in "dumpsys dnsresolver"

Change-Id: I8eff0fd6c8bfb96cb7baf4be701c2b0a14aa7213