In jsonprc2_test.go, binder.Bind starts a goroutine.
That goroutine begins to run during jsonrpc2.bindConnection, and
can race with the setting of conn.write in bindConnection.
This PR adds a sleep, which is a poor way to deal with the race, but
the least invasive change. Better ones include running the test
function after Dial returns, or adding a Connection.Ready method
to detect when initialization is complete.
be1ddf5f80