fix(test): do not throw on error.errors.map (denoland#12810)

In tests, the function to format errors would assume that any error with
a property `errors` would be an `AggregateError`, and therefore the
property `errors` would contain an error. This is not necessarily the
case.

runtime/js/40_testing.js

@@ -9,6 +9,7 @@
  const { serializePermissions } = window.__bootstrap.permissions;
  const { assert } = window.__bootstrap.util;
  const {
+ AggregateError,
  ArrayPrototypeFilter,
  ArrayPrototypePush,
  ArrayPrototypeSome,
@@ -297,7 +298,7 @@ finishing test case.`;
  }
 
  function formatError(error) {
- if (error.errors) {
+ if (error instanceof AggregateError) {
  const message = error
  .errors
  .map((error) =>