I think it might be sio-ctx.rs, but I'm not sure. Around the hang I see this output:

...
test [run-pass] ../src/test/run-pass/simple-obj.rs ... ok
test [run-pass] ../src/test/run-pass/sio-client.rs ... ignored
test [run-pass] ../src/test/run-pass/sio-ctx.rs ... 

It ends up passing, but it takes about a minute or so to get unstuck. I'm guessing it's a timeout or something we're running into.