Improve test readability

Add kMaxNumberOfClients to BufferHubDef. This variable will also be
needed when buffer state is changed according to
go/bufferhub-buffer-state-redesign

Test: BufferHubBuffer_test buffer_hub-test buffer_node-test
Bug: 112007999
Change-Id: I453438f3e2defa1fc93d1e74a282a38360f27694