Attempt to send stdio to the kernel before sending execute_reply

The Jupyter messaging docs state that all messages on the shell (request)
channel shall be ended with a `status: idle` message, and that: "This idle
status message indicates that IOPub messages associated with a given request
have all been received."

Previously we did not attempt to ensure this, which meant that in some cases the
`execute_result` message would be published after the `execute_reply` message
had been sent. It's still not exactly guaranteed by the `yield()`, but it's
something and it makes the `jupyter_kernel_test` tests pass.

Author: JamesWrigley

src/execute_request.jl

@@ -108,6 +108,13 @@ function execute_request(socket, kernel, msg)
 
         # flush pending stdio
         flush_all()
+        yield()
+        if haskey(kernel.bufs, "stdout")
+            send_stdout(kernel)
+        end
+        if haskey(kernel.bufs, "stderr")
+            send_stderr(kernel)
+        end
 
         undisplay(result, kernel) # dequeue if needed, since we display result in pyout
         @invokelatest display(kernel) # flush pending display requests