Handlebars: Add tests for {{! prettier-ignore}}

[dcyriller]

Description

This PR adds test cases for {{! prettier-ignore}} when it is above or before:

• a text element
• a mustache element
• a block element
• a comment element

@fisker it should help with #13690

Checklist

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[fisker]

Thanks for the test. It seems a little tricky to handle all these cases. Would you like to work on it?
If you do, please target the next branch, code changed in utils.