Add more fixture files with Lighthouse data

### Feature Description

We currently have 1 single fixture file for the `https:\\amp-wp.org` page.

We should extend this to have multiple pages across multiple real domains as fixture files, so that we can use the `--url` flag to choose one of multiple fixture files. These should include official pages from the AMP and/or WordPress project.

Ideally, we would have enough fixture coverage to have most or all of Lighthouse's audits be triggered for producing data.

To ensure we can easily maintain this, the fixture files should be generated via a script that we can trigger at a later point to update the fixture files.