Step 2 - Move `job_app_spec` to Jest

What does this MR do?

This MR is the second step to fixing the spec flakiness of the job_app component #55254 (closed).

• Step 1 - Fix job_app initialization and cleaning up of side-effects
• Step 2 - Move job_app_spec to Jest

Wait!

Don't merge this yet! Let's merge Step 1 into master then point this MR to master.

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry (Not needed since this is ~backstage)
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Refrences

• Part of #55254 (closed)