add custom interface to cache 'Mocha.interfaces'

lib/cli/run-helpers.js

@@ -196,9 +196,9 @@ exports.runMocha = async (mocha, options) => {
  * {@link handleRequires}), as it'll prevent interfaces from loading otherwise.
  * @param {Object} opts - Options object
  * @param {"reporter"|"interface"} pluginType - Type of plugin.
- * @param {Object} [map] - An object perhaps having key `key`. Used as a cache
- * of sorts; `Mocha.reporters` is one, where each key corresponds to a reporter
- * name
+ * @param {Object} [map] - Used as a cache of sorts;
+ * `Mocha.reporters` where each key corresponds to a reporter name,
+ * `Mocha.interfaces` where each key corresponds to an interface name.
  * @private
  */
 exports.validateLegacyPlugin = (opts, pluginType, map = {}) => {
@@ -226,12 +226,12 @@ exports.validateLegacyPlugin = (opts, pluginType, map = {}) => {
   // if this exists, then it's already loaded, so nothing more to do.
   if (!map[pluginId]) {
     try {
-      opts[pluginType] = require(pluginId);
+      map[pluginId] = require(pluginId);
     } catch (err) {
       if (err.code === 'MODULE_NOT_FOUND') {
         // Try to load reporters from a path (absolute or relative)
         try {
-          opts[pluginType] = require(path.resolve(pluginId));
+          map[pluginId] = require(path.resolve(pluginId));
         } catch (err) {
           throw createUnknownError(err);
         }