fix(curriculum):Add tests for universal selector in CSSHelp

[Iyeroo]

fix(curriculum):Added tests for the universal selector functionality in CSSHelp, including cases for existing and non-existent selectors.

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #64218

This test file verifies the behavior of the CSSHelp class with respect to universal selectors (*).

Specifically, it tests that:

1. CSS rules containing universal selectors are only matched when explicitly requested.

   • Example: A rule like span[class~="one"] *:first-of-type should match only when the exact selector is requested via getStyle.

2. Non-existent or disallowed selectors return null as expected.

Implementation notes:

• We mock the Document object to simulate a style sheet containing a universal selector rule.
• We define global.CSSRule to allow TypeScript compilation in a Node environment, since CSSRule is normally a browser global.
• getPropVal is verified to ensure that the helper method added by getStyle returns correct CSS property values.
• No real DOM or browser is used; the tests run entirely in Node.

These tests ensure that the fix preventing implicit matching of universal selectors works correctly.

[majestic-owl448]

please make these changes in #561