Add tests for alert colors on backgrounds in core component

[azerella]

As seen at: https://auds.service.gov.au/packages/core/tests/site/ we don't currently test the:

- au-color-error
- au-color-success
- au-color-warning
- au-color-info

colors against the AU-color-background-alt, AU-colordark-background-alt or AU-colordark-background