[CST] - Update AddFiles form tests

[pmclaren19]

Summary

Initially our team found that AddFilesForm had a bug after a user had added a file, when the user tried the select the Change File button and changed the file the component va-file-input reflected the new file but AddFilesForm was still showing both files. After investigation we found a larger bug which was that on Aug 13th the platform design team removed the ability to use the V1 version of the va-file-input component which is what your code on AddFilesForm was relying on, thus breaking upload section of the CST and only allowing users to upload one file at a time instead of multiple uploads.

Discussed this issue with the platform design team here. They are going to fix the issue by moving our component library back to the version that had the V1 component.

In this pr I am adding more tests so that in the future if this were to happen again we would catch it!

Related issue(s)

• Link to ticket created in va.gov-team repo
  [CST] Bug in file upload when replacing one file with another va.gov-team#96817
• Link to platform design ticket
  department-of-veterans-affairs/vets-website#0000
  - Link to epic if not included in ticket

Testing done

• Describe what the old behavior was prior to the change
• Describe the steps required to verify your changes are working as expected
• Describe the tests completed and the results
• _Exclusively stating 'Specs and automated tests passing' is NOT acceptable as appropriate testing
• Optionally, provide a link to your test plan and test execution records

Screenshots

Note: This field is mandatory for UI changes (non-component work should NOT have screenshots).

	Before	After
Mobile
Desktop

What areas of the site does it impact?

(Describe what parts of the site are impacted if code touched other areas)

Acceptance criteria

Quality Assurance & Testing

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Linting warnings have been addressed
• Documentation has been updated (link to documentation *if necessary)
• Screenshot of the developed feature is added
• Accessibility testing has been performed

Error Handling

• Browser console contains no warnings or errors.
• Events are being sent to the appropriate logging solution
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)

Authentication

• Did you login to a local build and verify all authenticated routes work as expected with a test user

Requested Feedback

(OPTIONAL) What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?