Fix FOSS specs broken by !58833 [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

!58833 (merged) was merged, but broke some FOSS specs because the code was moved to ee/ while the corresponding specs were not moved.

This MR fixes the FOSS diffs app_spec.js (FOSS_ONLY=1 yarn run jest spec/frontend/diffs/components/app_spec.js) by moving them to a new EE version of the diffs app_spec.js (yarn jest ee/spec/frontend/diffs/components/app_spec.js), where they should've been all along.

The first commit is a squashed commit of the changes from !58833 (merged), the second commit is the fix.

Screenshots (strongly suggested)

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 _____.
• 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

Related to #267612 (closed), #327620 (closed)