new audit: have a valid favicon.ico

#### Provide the steps to reproduce
1. Run LH on https://condescending-haibt-2d0d42.netlify.com/bug.html

#### What is the current behavior?
| URL | Description |
|------------------------------------------------------|-----------------------------------------------------------------------|
| /favicon.ico(condescending-haibt-2d0d42.netlify.com) | Failed to load resource: the server responded with a status of 404 () |


#### What is the expected behavior?
Favicon shouldn't be a violation

#### Environment Information
* Affected Channels: CLI, Node
* Lighthouse version: 5.6.0
* Chrome version: 80.0.3987.100
* Node.js version: 12.14.1
* Operating System: Windows

**Related issues**
https://github.com/GoogleChrome/lighthouse/issues/10381
https://github.com/GoogleChrome/lighthouse/pull/10190

Start by reproducing the Lighthouse CLI result for https://condescending-haibt-2d0d42.netlify.com/bug.html and reviewing related issue #10381 and pull request #10190. Define the audit's expected behavior from the missing /favicon.ico report; done means the audit handles this case consistently without treating the favicon problem as an unresolved violation.

Written by the indexing model from the issue text.