test(button): abstract test args to setup file

[dancormier]

This PR moves much of the button test arguments to button.test.setup.ts. This PR also introduces a11y testing for buttons that contain badges.

[!NOTE] This PR is inspired by failures noticed in button group test runs (see failing test run, PR)

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The accessibility failures found in the a11y test have been captured in Jira:

• light/dark modes: https://stackoverflow.atlassian.net/browse/A11Y-32
• light high contrast mode: https://stackoverflow.atlassian.net/browse/A11Y-33

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