Add unit test for merge_conflicts new vuex store

What does this MR do?

This MR fully covers the new vuex store with tests and adds some of the tests for the utility files, marking the todo ones.

Plan

Description	MR
Add new store	!55524 (merged)
Add basic tests for Utility, getters and mutation	👈 you are here
Wire the new store in the components and remove old one	!56756 (merged)

Screenshots (strongly suggested)

No visual changes

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because those are just some new tests.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team