spawn_blocking hangs in TestViewGenerator

[mrain]

Quoted from Brendon:

What's happen is:

• We create a TestViewGenerator stream
• call next().await to get the next (first view)
• This creates a future and calls poll_next code Chengyu linked abov
• This creates a future which calls TestView::genesis
• This future calls spawn_blocking
• When we await the spawned calc the future returns Pending
• Eventually the calculation concludes and we can re poll poll_next and that creates a brand new future calling TestView::genesis and thus the loop

https://github.com/EspressoSystems/espresso-sequencer/blob/29bb98ee2e2e1cc75dd1daa355e7d7918272044b/hotshot-testing/src/view_generator.rs#L649-L667