Add GetResolverStats to cache unit test

This unit test is for resolv_cache_get_resolver_stats().

Bug: 137169582
Test: cd packages/modules/DnsResolver && atest

Merged-In: I256910de5410a0df1ca3975c8eb62c50b338ed49
Change-Id: I13c7e2ff026104a1d0e4184e8672142ad9d76f05
(cherry picked from commit e8179b01)