Fix flaky tests due to onDnsEvent

Since not every test checks onDnsEvent, a test might receive a
DnsEvent which should have been catched by the previous test.

Modify ExpectDnsEvent() to drop unexpected DnsEvents until the
target DnsEvent is found.

Bug: 159439402
Test: delayed the resolver to send DnsEvents, and ran
      resolv_integration_test
Change-Id: Id06546fca9438f89aaa2af383b34b52b7e536e26