Fix a flaky test in buffer_hub-test

UDS-based PDX guarantees commands execution on the same channel. However,
where there are incoming commands from different channels, the execution
order can no longer be guaranteed as the service basical holds a epoll
on sockets from different FDs.

The original test we put up is flaky for the following commands:

c1->ReleaseAsync
p->CreateConsumer

Not that the ReleaseAsync call may actually happen after a new consumer
gets created, which prevents the buffer from being fulling released back
to the producer.

To de-flaky the test, simplily poll the producer after the release to
confirm that the buffer is indeed released before moving on.

Bug: 70298522
Bug: 70046255
Test: buffer_hub-test
Change-Id: Ia3987642a40f62e7d151edbdf0f11dd9bee57223