chore: add subdirs.test.js that requires in subdirectories

Re: #8322

Author: vkarpov15

test/files/sample.esm renamed to test/files/sample.js

@@ -1,3 +1,2 @@
 --reporter dot
---ui bdd
---recursive
+--ui bdd

test/mocha.opts

@@ -0,0 +1,15 @@
+'use strict';
+
+const fs = require('fs');
+
+let files = fs.readdirSync(`${__dirname}/docs`);
+
+for (const file of files) {
+  require(`./docs/${file}`);
+}
+
+files = fs.readdirSync(`${__dirname}/helpers`);
+
+for (const file of files) {
+  require(`./helpers/${file}`);
+}