The calls are made asynchronous, so you need to use callback functions to chain them.
it('should respond to GET with added items', function(done) {
var agent = request(app);
agent.post('/player').type('json').send({name:"Messi"}).end(function(){
agent.post('/player').type('json').send({name:"Maradona"}).end(function(){
agent.get('/player')
.set("Accept", "application/json")
.expect(200)
.end(function(err, res) {
res.body.should.have.property('items').with.lengthOf(2);
done();
});
});
});
});