Minor refactor value streams dashboard mock data

What does this MR do and why?

Updates mock data used for testing the value streams dashboard:

• Updates mockMetrics to take an object with keys to specify the metric and relevant value
• Adds mockChartDataValues to generate chart data
• Removes unnecessary undefined data

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Run the jest tests locally

$ yarn jest analytics/dashboards

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.