Fix spec violations in memory_graph_spec

What does this MR do and why?

In regards of epic &5102, this MR fixes the following spec violations in spec/frontend/vue_shared/components/memory_graph_spec.js #391122 (closed):

• Use of Vue.extend

How to set up and validate locally

Run tests:

yarn jest spec/frontend/vue_shared/components/memory_graph_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.

Closes #391122 (closed)