Add WPT to determine whether reception is interleaved

[jan-ivar]

Write WPT write a test that sends a little bit of data on stream A followed by a lot of data on stream B, then see if we can read out the second stream before the first is done.

[aboba]

If writer.write() is used instead of await writer.write() then reception appears to be interleaved. When I made this substitution in the WebCodecs-echo sample, the glass-glass latency went down and reordering was observed on the receiver side (requiring a re-ordering buffer).