Testing: Add e2e test for basic ContrastChecker functionality

[himanshupathak95]

Closes: #68853

What?

Add a basic end-to-end test for the ContrastChecker component that verifies the low-contrast warning appears immediately when the text and background colors have insufficient contrast.

Why?

After enhancing the ContrastChecker component in #68799, it's important to ensure its core functionality of detecting and warning about low-contrast color combinations works reliably.

Testing Instructions

• Run npm run test:e2e -- editor/various/contrast-checker.spec.js to execute the test
• Verify that it is passing and debug (if necessary) to check the working

Screencast

Screen.Recording.2025-01-24.at.13.25.25.mov

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Co-authored-by: himanshupathak95 <abcd95@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: afercia <afercia@git.wordpress.org>
Co-authored-by: juanfra <juanfra@git.wordpress.org>

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[afercia]

After the first test gets refined, it would be nice to add another test as @juanfra mentioned in #68853 (comment)
To my understanding, it would be basically: set only the text color to white and see if the contrastChecker warns against the default background color.

[Mamaduka]

I think this can be located using the following getByRole selector.

await page.getByRole( 'option', { name: 'Black' } ).click();

[Mamaduka]

Same.

[Mamaduka]

@himanshupathak95, thanks for refactoring. Left two minor notes. Let's add the test case @afercia mentioned and this should be good to merge.

I think we can just use test steps and test everything in a single test.