Fix some flaky tests due to DnsTlsFrontend::queries()

DnsTlsFrontend updates its query count after sending DNS answers
to the DnsResolver. There is a small time gap betwen them. If tests
check the query count before DnsTlsFrontend updates it, the tests
will fail. Therefore, we use waitForQueries() to fix it.

Bug: 232047120
Test: ran QueryTlsServerTimeout 100 times
Test: ran DotQuickFallback 100 times
Change-Id: I4f7968aab99654854b8c22f881c8a418fef3653f