[Tests] Icon - Enhance Unit Tests

[TimRoe]

Description of Change

Added 8 new Icon unit tests around the fill property testing:

• 4 categories:
  • Single string (does not change with light/dark mode)
  • "default" colors
  • "base" colors
  • Custom light/dark mode color object
• Light/dark mode for each

Made minor adjustments to existing tests to simplify and/or improve them.

Testing

yarn tests passes with all new Icon tests. Draft tested different expect values caused failures as expected.

PR Checklist

Code reviewer validation:

• General
  • PR is linked to ticket(s)
  • PR has changelog label applied if it's to be included in the changelog
  • Acceptance criteria:
    • All satisfied or
    • Documented reason for not being performed or
    • Split to separate ticket and ticket is linked by relevant AC(s)
  • Above PR sections adequately filled out
  • 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
  • Tests are included if appropriate (or split to separate ticket)
  • New functions have proper TSDoc annotations

Publish

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