Intermittent failure with TypeError: Network error occurred 

This happened with `/html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*)` but I strongly suspect it's not about that specific test.


```
  ▶ Unexpected subtest result in /html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*):
  │ FAIL [expected PASS] idl_test setup
  └   → promise_test: Unhandled rejection with value: object "TypeError: Network error occurred"
```