test(overlays): add Axe tests for color contrast

[amandaejohnston]

Issue number: Internal

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What is the current behavior?

• The action-sheet, alert, and toast components have tests for Axe violations, but the color-contrast rule is disabled.
• The select-popover component does not test for Axe violations.
• For all of the above plus loading, dark mode test for color contrast are not included.

What is the new behavior?

• Existing Axe tests pulled out into separate configs blocks that test both light and dark themes. Markup has also been converted to use page.setContent instead of page.goto to support automatic theme switching.
• color-contrast rule re-enabled on action-sheet, alert, and toast. The rule was disabled due to contrast issues with the page content covered up by the backdrop, but with the new setup, the overlay is the only thing on the page.
• For select-popover, the Axe test has been limited to only color contrast testing for now. The component had several Axe failures that will be addressed separately to keep scope down. (We didn't previously have any Axe testing on this component, which is why this wasn't caught.)
• HTML file for the loading a11y test removed, since we're no longer using it.

Does this introduce a breaking change?

• Yes
• No

Other information

[liamdebeasi]

Looks good! One non-blocking suggestion and one question.

[liamdebeasi]

Great work!