stream_send_recv_stress tests: wait for threads to finish

[RalfJung]

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.

[rustbot]

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