Avoid multiple inheritance in ResolverTest

This is better modeled by an HAS-A relationship, and multiple
inheritance can cause a good deal of confusion. For instance, it wasn't
clear why GTest wouldn't call DnsResponderClient::SetUp() after deleting
ResolverTest::SetUp().

Test: atest resolv_integration_test
Change-Id: I69f5e53917a491f9d7035c0ff2399077f46e380c