Are these good unit tests? I've been lazy about doing good unit testing, but I'm trying to change that.

users.js

'use strict';
var db = require('mongoose').connection;
var credential = require('credential');
var User = require('./models/user');  // the mongoose model

exports.createUser = function createUser(userAttrs, callback) {
	db.once('open', function () {
		User.findOne({ username: userAttrs.username }, 'username', function (err, person) {
			if (err) return callback(err);
			if (person) {
				return callback(new Error('User "' + userAttrs.username + '" already exists'));
			} else {
				credential.hash(userAttrs.password, function (err, hash) {
					if (err) return callback(err);
					userAttrs.password = hash;

					var user = new User(userAttrs);
					return user.save(function (err, user) {
						if (err) return callback(err);
						return callback(null, user);
					});
				});
			}
		});
	});
};

usersTest.js

'use strict';
var test = require('tape');
var sinon = require('sinon');
var rewire = require('rewire');

var users = rewire('../lib/users');

users.__set__('db', { once: sinon.stub().yields() });

test('Creating a user throws error if username is already taken', function (t) {
	users.__set__('User', { findOne: sinon.stub().yields(null, {})});

	users.createUser({ username: 'andrew', password: 'password'}, function (err, user) {
		t.equal(err.message, 'User "andrew" already exists');
		t.end();
	});
});

test('Creating a user returns the user if it does not exist', function (t) {
	function UserStub(attrs) {
		this.save = sinon.stub().yields(null, attrs);
	}
	UserStub.findOne = sinon.stub().yields(null, undefined);

	users.__set__('User', UserStub);
	users.__set__('credential', { hash: sinon.stub().yields(null, { hash: 'password' }) });

	users.createUser({ username: 'andrew', password: 'password' }, function (err, user) {
		t.error(err);
		t.equal(user.username, 'andrew');
		t.deepEqual(user.password.hash, 'password');
		t.end();
	});
});

Strive not to mix conditionals with asynchronous callback nesting, as that's very prone to cause confusion to your future self, and maybe even your present self.

One way to do that is to use more guard clause style. E.g

if (foo) {
  done(new Error('foo!')); return;
}

Instead of (in your code example)

if (foo) {
  done(new Error('foo!')); return;
} else {
  bar();
}

Other than that, your tests seem cool! Keep it up. Writing tests mostly comes down to practice!