dnsdist: Prevent spurious failures of the async unit tests

dnsdist: Prevent spurious failures of the async unit tests

The events should be triggered either almost immediately or after
10 ms, but we have seen many spurious failures on our CI, likely
because the box is overloaded, so sleep for up to 100 ms to be sure.
I managed to reproduce the issue locally by running this command in
parallel of the tests, for reference: `stress --cpu <number of HT cores>`.