Add test coverage for DnsResolverService::dump()

The thing that calling the dump() at the end of every integration test
can help reduce the possibility of resolver service crash when dump
is being called. This is important especially when bugreport is being
taken.

cd packages/modules/DnsResolver && atest

Change-Id: Iec114a4de4082314496c9f7a3769db51ef42d857