Even thought the first rxresp implies that the backend fetch must have
completed, it is still possible to observe a streaming hit.

Since resp.is_streaming merely looks at the presence of a boc, and not
whether its state is lower than FINISHED, waiting until we see the fetch
end in the logs and forcing a new client session should reduce the race
window in those test cases.

We could also try to settle on the value of resp.is_streaming when we
hit an object in the cache or resume from a backend fetch and change the
semantic to whether resp was streaming when VCL entered the subroutine.

By that I mean, with a proper check of the boc state.