Work around issue with Node v14 and Puppeteer

Use the system version of Chrome if the developer has Node v14
installed, to work around an upstream issue with Puppeteer (see code
comment).

src/karma.config.js

@@ -7,7 +7,15 @@ const envify = require('loose-envify/custom');
 const glob = require('glob');
 
 let chromeFlags = [];
-process.env.CHROME_BIN = require('puppeteer').executablePath();
+
+if (process.version.startsWith('v14.')) {
+  // See https://github.com/puppeteer/puppeteer/issues/5719
+  console.warn(
+    'Using system Chrome instead of Puppeteer due to issue with Node 14'
+  );
+} else {
+  process.env.CHROME_BIN = require('puppeteer').executablePath();
+}
 
 // On Travis and in Docker, the tests run as root, so the sandbox must be
 // disabled.