You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: iter_test.go
    path of file: ./repos/go-ethereum/p2p/enode
    
    the code of the file until where you have to start completion: // Copyright 2019 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ether
    
func testMixerClose(t *testing.T) {
	mix := NewFairMix(-1)
	mix.AddSource(CycleNodes(nil))
	mix.AddSource(CycleNodes(nil))

	done := make(chan struct{})
	go func() {
		defer close(done)
		if mix.Next() {
			t.Error("Next returned true")
		}
	}()
	// This call is supposed to make it more likely that NextNode is
	// actually executing by the time we call Close.
	runtime.Gosched()

	mix.Close()
	select {
	case <-done:
	case <-time.After(3 * time.Second):
		t.Fatal("Next didn't unblock on Close")
	}

	mix.Close() // shouldn't crash
}