Rollup merge of #156365 - RalfJung:stream_send_recv_stress, r=nia-e

stream_send_recv_stress tests: wait for threads to finish

These tests currently fail in Miri (when run with nextest) because all they do is spawn a lot of threads that will do stuff, but they don't wait for the threads to actually finish. Miri by default errors when there are background threads lingering when `main` is done (since that can indicate a leak, and since it makes it impossible to check for memory leaks). Miri gives background threads a bit of time to finish when `main` is done, but for these tests that's nowhere near enough since basically the entire test runs after `main` is done.

Outside Miri, this could also still mean that the test doesn't actually run to completion, it might get abort when `main` finishes.

So let's use `thread::scope` to ensure all threads are done before the test is considered done.