Add more unit test for bufferhub

TestAsyncStateTransitions
TestZeroConsumer
TestMaxConsumers
TestCreateConsumerWhenBufferGained
TestCreateConsumerWhenBufferPosted
TestCreateConsumerWhenBufferReleased

This also renames the test to buffer_hub-test to match the name scheme
we are using elsewhere (i.e. buffer_hub_xxx instead of bufferhub_xxx and
yyy-test vs yyy_tests).

Bug: 68152849
Test: buffer_hub-test
Change-Id: I18ed32862bbee207e2751139599eff4f6f6e8618