E2E tests: migrate MR vue qa-selectors to testid

What does this MR do and why?

Part of gitlab-org/quality/quality-engineering/team-tasks#1825 (closed)

Migrates some Vue data-qa-selectors to data-testid for the following MR-related files in the E2E tests:

• qa/qa/page/merge_request/show.rb

I haven't migrated all of the selectors as the MR would have gotten too large.

How to set up and validate locally

Since this affects many E2E tests, probably best to validate using pipeline results.

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.