Fix TlsServerRevalidation flaky

The test becomes flaky because tls.clearQueries() is called without
tls.waitForQueries(). A recent change has made DnsTlsFrontend take
extra 50 ms to wait for another query from DoT validation, the test
will fail if tls.clearQueries() is called while DnsTlsFrontend is
still waiting for the query.

Bug: 188153519
Bug: 191147599
Test: resolv_integration_test64 --gtest_filter="*TlsServerRevalidation"
      --gtest_repeat=100 --gtest_break_on_failure
Change-Id: Ieb81a407c2a9c6aa119ab4499a93a9ffd663b716