Add missing QA selectors for Vue group members view

What does this MR do?

Adds missing QA selectors to new Group members view that was converted from HAML to Vue in &4233 (closed)

This MR does not yet default the feature flag to on as that will be done closer to the cut-off for %13.6. To make sure these E2E tests pass I did run the QA pipeline with the feature flag defaulted to on before turning it off again: https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/pipelines/213350423

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Developer facing change
• [-] 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