HTML tests: change subtle color for tests that customized background color

[compulim]

Related to #4650.

Changelog Entry

Fixed

• Related to #4650. Added automated accessibility check using axe-core
  • HTML test: changed contrast ratio in tests that use different background colors, by @compulim, in PR #XXX

Description

Our default subtle color is good for white background. We are not changing our default subtle color.

However, for some tests that changed background colors, such as tests that use focusManagement.css, we need to change subtle color to #666 so contrast looks good.

For example, the subtle color used in placeholder are not 4.5:1 in tests that changed focus color to yellow.

Design

Specific Changes

• Modify tests that use a colored background to pass styleOptions.subtle: '#666'

• I have added tests and executed them locally (Relies on CI)
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)