Batch comments Pinia migration 5/5: migrate getters spec

What does this MR do and why?

This MR is a part of Migrate MR Batch Comments store from vuex to Pinia (#512721 - closed) & Migrate Diffs and Notes Vuex stores to Pinia (#474653 - closed) efforts.

In this MR we're migrating batch comments getters spec to Pinia. This is the last migration MR for batch comments. Components are going to be migrated in another effort.

Step	MR
Copy store	!176485 (merged)
Migrate batch comments store to Pinia	!176486 (merged)
Migrate batch comments actions spec	!176487 (merged)
Migrate batch comments mutations spec	!176488 (merged)
Migrate batch comments getters spec	You are here