Since you are using chaning feature of webdriverjs it is wrong to use callback in waitFor
function.
Function saveScreenshot
is also chained. So the proper way would be the following:
it('should see the correct title', function(done) {
client.url('http://localhost/mysite/')
.getTitle( function(err, title){
expect(err).to.be.null;
assert.strictEqual(title, 'My title page' );
})
.waitFor( selector, 2000)
.saveScreenshot( "./ExtractScreen.png" )
.call(done);
});