chore: upgrade to Mocha v8 (#5997)

* chore: upgrade to Mocha v8

Mocha v8 has some nice improvements and also unlocks the potential
to have parallel test runs which may speed up CI.

mocha-config/puppeteer-unit-tests.js

@@ -18,9 +18,10 @@ const base = require('./base');
 
 module.exports = {
   ...base,
-  file: ['./test/mocha-utils.js'],
+  require: ['./test/mocha-utils.js'],
   spec: 'test/*.spec.js',
+  parallel: process.env.CI && !process.env.COVERAGE,
   // retry twice more, so we run each test up to 3 times if needed.
-  retries: 2,
+  retries: process.env.CI ? 2 : 0,
   timeout: 25 * 1000,
 };