[Test] Deflake GetAddrInfoV4_deferred_resp

Instead of waiting for 100m before launching the second thread, check
the number of queries arrived at dnsresponder to ensure that DNS sent
by the first thread precede DNS sent by the second thread.

Bug: 286832451
Bug: 289220389
Change-Id: I88a9b6525dd4575eb1b1a940794c589b490edfeb
Test: atest --rerun-until-failure=500
      resolv_integration_test:ResolverTest#GetAddrInfoV4_deferred_resp