[Tests] Add Loading indicator unit tests

[TimRoe]

Description of Change

Added unit tests for Loading indicator component including:

• Basic tests
• Accessibility
• Light/dark mode
• With/without children

Testing

yarn test passes all new tests and failed as expected when changing expect statements to inaccurate values.

PR Checklist

Code reviewer validation:

• General
  • [x] PR is linked to ticket(s)
  • [x] PR has changelog label applied if it's to be included in the changelog
  • [x] Acceptance criteria:
    • All satisfied or
    • Documented reason for not being performed or
    • Split to separate ticket and ticket is linked by relevant AC(s)
  • [x] Above PR sections adequately filled out
  • [x] If any breaking changes, in accordance with the pre-1.0.0 versioning guidelines: a CU ticket has been created for the VA Mobile App detailing necessary adjustments with the package version that will be published by this ticket
• Code
  • [x] Tests are included if appropriate (or split to separate ticket)
  • [x] New functions have proper TSDoc annotations

Publish

Changes do not warrant a new version per the versioning guidelines as they are only unit tests.