[css-color] add test for LinkText and dark color-scheme

[mayank99]

Changes to system-color-consistency.html:

1. Added a new test for LinkText.
   • This test is now actually accurate after Default link colors lack sufficient contrast in the dark color scheme WebKit/WebKit#17512.
2. All tests are now run twice, once for color-scheme: light (default) and once for color-scheme: dark

Review with whitespace off for proper diff.

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Note: I would have liked to also test for VisitedText but I'm not sure how to. <a href=""> should technically work but getComputedValue doesn't return the correct value (likely for security reasons).

[svgeesus]

getComputedValue doesn't return the correct value (likely for security reasons)

Exactly for that reason, yes.

User Agents may, to mitigate privacy and security risks such as fingerprinting, elect to return fixed values for the used value of system colors which do not reflect customisation or theming choices made by the user.
https://drafts.csswg.org/css-color-4/#css-system-colors

[svgeesus]

LGTM thanks for this addition