Puppeteer engine scripts don't seem to be running

After changing the engine on our configuration file from chromy to puppeteer, it doesn't seem like the engine_scripts files are running i.e. onReady and clickAndHoverHelper (the console.log in these files are never printed, and clickSelector, hoverSelector etc don't result in the expected actions). Any ideas why this may be?

```
const backstop = require('backstopjs');
const path = require('path');
const fs = require('fs');
const argv = require('yargs').argv;

let backstopMode = 'test';
let allscenarios = [];
let dns = argv.e;
// if component is specified,retrieves scenarios from the component json file
if (argv.c != null) {
allscenarios = require(path.resolve(__dirname, `./tests/${argv.c}.json`)).scenarios;
} else {
// if no component is specified,build scenarios from all components from scenarios folder
const files = fs.readdirSync(path.resolve(__dirname, 'tests'));
files.forEach((file) => {
const iconScenarios = require(path.resolve(__dirname, `./tests/${file}`)).scenarios;
allscenarios = allscenarios.concat(iconScenarios);
});
}
// set backstop command to 'test' if user inputs reference option
if (argv.r == 'reference') {
backstopMode = 'reference';
}

// Use default env if env option is not specified
const default_environment = 'http://localhost:4001';
if (!argv.e) {
dns = default_environment;
}

// Replace URL in all scenarios
const tempScenarios = JSON.stringify(allscenarios).replace(/\$dns/g, dns);
allscenarios = JSON.parse(tempScenarios);

// building backstop config file for headless chrome
backstop(backstopMode, {
config: {
id: 'test',
viewports: [
{
name: 'desktop',
width: 1920,
height: 2500,
},
],
scenarios: allscenarios,
paths: {
engine_scripts: path.resolve(__dirname, 'utils'),
bitmaps_reference: path.resolve(__dirname, 'test-output/chrome/expected_result'),
bitmaps_test: path.resolve(__dirname, 'test-output/chrome/actual_result'),
html_report: path.resolve(__dirname, 'test-output/chrome/html_report'),
ci_report: path.resolve(__dirname, 'test-output/chrome/ci_report'),
},
engine: 'puppeteer',
engineOptions: {
args: ['--no-sandbox'],
},
report: ['browser', 'CI'],
debug: false,
delay: 500,
misMatchThreshold: 0.1,
},
asyncCaptureLimit: 2,
asyncCompareLimit: 25,
}).catch((e) => {
console.log(e);
process.exit(1);
});
```