[web.dev] Incorrectly flagging cross-origin links as not containing noopener/noreferrer (Best Practices)

#### Provide the steps to reproduce
1. Run LH on https://willstocks.co.uk
2. View the results
3. Scroll to Best Practices

#### What is the current behavior?

Under "Best Practices", I'm seeing the following:

However, when I check my markup, I'm seeing that `noopener` and `noreferrer` are infact defined alongside `bookmark`.

The markup that is being complained about is:
```html

```
#### What is the expected behavior?

If the link to a cross-origin domain contains the recommended `rel` attributes, surely this shouldn't flag up as an issue?

#### Environment Information
* Affected Channels: https://web.dev/measure
* Lighthouse version: 5.6.0
* Chrome version: 80.0.3987.122
* Node.js version:
* Operating System: W10 Enterprise 1909

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