Karma to Jest migration for monitor chart components specs

What does this MR do?

Moved 3 chart specs to Jest from Karma:

• /monitoring/charts/column_spec.js
• /monitoring/charts/empty_chart_spec.js
• /monitoring/charts/single_stat_spec.js

Using the Karma to Jest script: https://gitlab.com/gitlab-org/frontend/playground/migrate-karma-to-jest

Unfortunately time_series_spec.js is not as trivial, not migrated in this MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content