Migrate diff_line_gutter_content_spec to jest

What does this MR do?

Migrates diffs/components/diff_line_gutter_content_spec.js from Karma to Jest.

It also consolidates some mock data that was duplicated.

Screenshots

N/A this is a backstage change.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content