Refactor issuables list to Vue, part 2 (tests)

What is left to do

• Make a CE port of this MR and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15091
• https://gitlab.com/gitlab-org/gitlab-ce/issues/66655 needs to be resolved before the build passes. If the issues API exposes a reference_path property on issue objects, no code change should be needed - just rerun the build and it should pass.

What does this MR do?

• Adds Jest tests for the issuables list component
• Updates tests for changed components
• Fixes RSpecs to wait until the issuables list loads

Maintainer: How to merge

This is part 2 of a pair of MRs. Part 1 is here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15091

Part 2 contains tests and code changes to satisfy tests. Part 1 contains business logic.

Neither part has EE-specific code, and can be cleanly backported to CE.

TO MERGE:

1. Merge this MR into mh/vue-issuables-list-ee. This MR is based on that branch, so the pipelines on Part 1 should then pass.
2. Merge Part 1 into master.
3. 🐝 happy.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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