Convert ThroughputChart tests to createMockApollo

What does this MR do and why?

Stop directly modifying the Vue data and instead mock the GraphQL that the component receives. This is a prerequisite for updating our codebase to Vue3.

How to set up and validate locally

yarn jest ee/spec/frontend/analytics/merge_request_analytics/components/throughput_chart_spec.js

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.

Related to #390987 (closed)