`usa-alert` - visual diffing and regression testing

[ethangardner]

We have been manually creating the initial visual diff files, e.g. e2e/components/usa-banner/usa-banner.spec.ts-snapshots, by using core as a reference. Now that usa-alert has an initial code contribution, it needs some additional attention.

• Render each variant of the alert component from USWDS core on a page, take a screenshot of the element, and add each variant to the appropriate screenshot folder.
• Add relevant tests to the e2e directory