Chore: convert StubModuleResolver in config tests to ES6 class (#8265)

Unlike the modules described in #8231, StubModuleResolver is not exposed as part of the public API, so this is not a breaking change.

tests/lib/config/config-file.js

@@ -123,17 +123,15 @@ function createStubModuleResolver(mapping) {
      * @constructor
      * @private
      */
-    function StubModuleResolver() {}
+    return class StubModuleResolver {
+        resolve(name) { // eslint-disable-line class-methods-use-this
+            if (mapping.hasOwnProperty(name)) {
+                return mapping[name];
+            }
 
-    StubModuleResolver.prototype.resolve = function(name) {
-        if (mapping.hasOwnProperty(name)) {
-            return mapping[name];
+            throw new Error(`Cannot find module '${name}'`);
         }
-
-        throw new Error(`Cannot find module '${name}'`);
     };
-
-    return StubModuleResolver;
 }
 
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