fix(backend): ESM/CJS interop is sad

Fix ESM/CJS interop issue in unit tests with a hack that checks the
class name as a string instead of just doing an `instanceof` check. This
is assuming that the "rewriter must be a PermissionRewriter" error I'm
seeing on GitHub is because there are two different "copies" of
PermissionWriter due to ESM/CJS interop issues. If this is not the case,
then you're not reading this commit message on `main`; if this commit is
on `main` then that's exactly what went down.

src/backend/src/services/auth/PermissionService.js

@@ -1191,7 +1191,10 @@ class PermissionService extends BaseService {
      * @param {PermissionRewriter} rewriter - The permission rewriter to register
      */
     register_rewriter (rewriter) {
-        if ( ! (rewriter instanceof PermissionRewriter) ) {
+        const is_permission_rewriter = rewriter instanceof PermissionRewriter
+            // Hack for ESM/CJS interop issue in unit tests.
+            || rewriter?.constructor?.name === 'PermissionRewriter';
+        if ( ! is_permission_rewriter ) {
             throw new Error('rewriter must be a PermissionRewriter');
         }