fix(challenges): add missing test to check for css class

[Nirajn2311]

ISSUES CLOSED: #254

Description

I have added the below test

        {
          "text":
            "Define a class named <code>silver-background</code> within the <code>style</code> element and assign the value of <code>silver</code> to the <code>background-color</code> property.",
          "testString":
            "assert(code.match(/\\.silver-background\\s*{\\s*background-color:\\s*silver;\\s*}/), \"Define a class named <code>silver-background</code> within the <code>style</code> element and assign the value of <code>silver</code> to the <code>background-color</code> property.\");"
        }

The test checks if there is a silver-background class in the style element and makes the sure the below code doesn't pass the test.

  div { /* This should not pass the tests */
    background-color: silver;
  }

Pre-Submission Checklist

• [x] Your pull request targets the dev branch.
• [x] Branch starts with either fix/, feature/, or translate/ (e.g. fix/challenge-tests)
• [x] All new and existing tests pass the command npm test.
• [x] Use npm run commit to generate a conventional commit message. Learn more here: https://conventionalcommits.org/#why-use-conventional-commits
• [x] The changes were done locally on your machine and NOT GitHub web interface. If they were done on the web interface you have ensured that you are creating conventional commit messages.

Checklist:

• [x] Tested changes locally.
• [x] Addressed currently open issue (replace XXXXX with an issue no in next line)

Closes #254

[scissorsneedfoodtoo]

@Nirajn2311, thank you for this fix! Merging now.

And thank you for helping to QA this, @joshalling.