Fix CSS variable escape test strings

[Mirochill]

Summary

• make the CSS variable escaped-hex fixture use a single CSS escape
• mark the escaped-character CSS variable fixture as a raw string to avoid Python invalid escape warnings

Fixes #3973.

Validation

• git diff --check
• Not run locally

[github-actions]

👈 Launch a binder notebook on branch Mirochill/ipywidgets/fix-3973-css-var-escapes