React + Jest + Enzyme with non-webpack app

.babelrc

{
    "plugins": [
        "transform-class-properties",
        "transform-react-jsx"
    ],
    "presets": [
        "stage-2",
        ["env", {
            "targets": {
                "browsers": [
                    "last 2 versions",
                    "not last 2 ie versions",
                    "last 1 ie versions"
                ]
            }
        }]
    ]
}

.setup.js

const Enzyme = require("enzyme");
const EnzymeAdapter = require("enzyme-adapter-react-16");

// Setup enzyme's react adapter
Enzyme.configure({ adapter: new EnzymeAdapter() });

package.json

{
    "name": "jest",
    "version": "1.0.0",
    "main": "index.js",
    "license": "MIT",
    "dependencies": {
        "react": "^16.4.0",
        "react-dom": "^16.4.0"
    },
    "devDependencies": {
        "babel": "^6.23.0",
        "babel-plugin-transform-react-jsx": "^6.24.1",
        "babel-preset-env": "^1.7.0",
        "babel-preset-stage-2": "^6.24.1",
        "enzyme": "^3.3.0",
        "enzyme-adapter-react-16": "^1.1.1",
        "jest": "^23.1.0"
    },
    "jest": {
        "verbose": true,
        "collectCoverage": true,
        "setupTestFrameworkScriptFile": "<rootDir>/.setup.js",
        "moduleDirectories": [
            "node_modules"
        ],
        "coverageReporters": [
            "text",
            "html",
            "lcov"
        ]
    }
}

Sample.js

(function (factory) {
    if (typeof module === 'object' && module.exports) {
        module.exports = function (root, jQuery) {
            if (jQuery === undefined) {
                if ( typeof window !== 'undefined' ) {
                    jQuery = require('jquery');
                } else {
                    jQuery = require('jquery')(root);
                }
            }

            factory(jQuery);

            return jQuery;
        };
    } else {
        factory(jQuery);
    }
}(function ($) {
    window.app = { Sample: {} };

    window.app.Sample = class Demo extends React.Component {

        state = { count: 0 };

        constructor(props) {
            super(props);
        }
    
        incrementCounter() {
            this.setState({ count: this.state.count + 1 });
        }

        render() {
            return (
                <div>
                    <a onClick={this.incrementCounter.bind(this)}>Click </a>
                    <span>{this.state.count}</span>
                </div>
            );
        }

    };
}));

Sample.test.js

const React = require('react');
const { shallow } = require('enzyme');

const Sample = require('./Sample');

beforeAll(function () {
    window.React = React;
});

describe('Demo test', function () {
    it('Module is properly exported and can be consumed by Jest', function () {
        expect(Sample).toBeDefined();
    });

    it('Should render', function () {
        const sample = shallow(<Sample />);
        expect(sample.find('span')).toHaveLength(1);
    });
});