test: refactor how spec files are collected (#23774)

spec/static/get-files.js

@@ -0,0 +1,15 @@
+async function getFiles (directoryPath, { filter = null } = {}) {
+  const files = [];
+  const walker = require('walkdir').walk(directoryPath, {
+    no_recurse: true
+  });
+  walker.on('file', (file) => {
+    if (!filter || filter(file)) {
+      files.push(file);
+    }
+  });
+  await new Promise((resolve) => walker.on('end', resolve));
+  return files;
+}
+
+module.exports = getFiles;