Test: Rewrite ResolverTest.KeepListeningUDP

Rewrite the test for more test coverage, including
- resnsend is tested as well.
- a test scenario where DNS response is received during the second
  iteration of DNS servers.

Bug: 271405311
Test: resolv_integration_test
Change-Id: Idd93d30987a717d875723e598fb2435b620bcec2