mocha didn't work when using with fs.writeFile or fs.readFile

test.js

var fs = require('fs'),
    os = require('os');

var assert = require('assert'),
    expect = require('expect.js'),
    should = require('should');

describe('Test on I/O', function() {
  it('write test', function() {
    fs.writeFile('./text.txt', "Hello Node.js" + os.EOL, function(err) {
      if (err) {
        throw "Unable to read file";
      }
      expect(3).to.equal(4);    // expect didn't work
      assert.equal(3, 4);       // assert didn't work
      (3).should.be.exactly(4); // should didn't work
    });
  });

  it('read test', function() {
    fs.readFile('./file.txt', function(err, data) {
      if (err) {
        throw "Unable to read file";
      }
      expect(3).to.equal(4);    // expect didn't work
      assert.equal(3, 4);       // assert didn't work
      (3).should.be.exactly(4); // should didn't work
    });
  });
});