dnsdist: Remove a racy test in the AsynchronousHolder unit tests

dnsdist: Remove a racy test in the AsynchronousHolder unit tests

We are adding an expired event so the worker thread of the
AsynchronousHolder can pick it up immediately, even before we come
back from the call to push(), which leads to a racy test.
This was observed on GitHub Actions when running with TSAN:
```
FAIL: testrunner
================

Running 170 test cases...
test-dnsdistasync.cc(156): error: in "test_dnsdistasync/test_AddingExpiredEvent": check !holder->empty() has failed

*** 1 failure is detected in the test module "unit"
FAIL testrunner (exit status: 201)
```