resolv: add some tests for resolver cache

Add some tests for res_cache to ensure the basic functions work.

Bug: 134897288
Test: cd systen/netd && atest
Test: in cuttlefish, ran resolv_unit_test --gtest_repeat=200 --gtest_filter="ResolvCacheTest.*"
Change-Id: I0347952debf2b1519afc25dea13505fd6e9a49e8