sample

sample.js

casper.test.begin("Pre executed two or more R cells and Rmarkdown cells to be executed using Run All", 12, function suite(test) {

    var x = require('casper').selectXPath;
    var github_username = casper.cli.options.username;
    var github_password = casper.cli.options.password;
    var rcloud_url = casper.cli.options.url;
    var functions = require(fs.absolute('basicfunctions'));
    var input_code = "50+50" ;
    var expected_result = "100";
    var errors = [];

    casper.start(rcloud_url, function () {
        casper.page.injectJs('jquery-1.10.2.js');
    });
    casper.wait(10000);

    casper.viewport(1024, 768).then(function () {
        functions.login(casper, github_username, github_password, rcloud_url);
    });

    casper.viewport(1024, 768).then(function () {
        this.wait(9000);
        console.log("validating that the Main page has got loaded properly by detecting if some of its elements are visible. Here we are checking for Shareable Link and Logout options");
        functions.validation(casper);

    });
    
//Create a new Notebook.
functions.create_notebook(casper);
    
casper.then(function(){
this.click({type:'xpath', path:"/html/body/div[3]/div/div[2]/div/div[3]/div[1]/div/span/i"});//creating 1st R cell
this.wait(3000);
var form =  document.evaluate(".//*[@id='part1.R']/div[2]/div[2]/select", document, null,XPathResult.FIRST_ORDERED_NODE_TYPE);
});    
casper.wait(10000);
    
casper.run(function () {
test.done();
});
});
    

You should use

this.wait(3000, function(){
  this.echo(this.evaluate(function(){
    return !!document.evaluate("//*[@id='part1.R']/div[2]/div[2]/select", document, null,XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue; });
  }));
});