Make tests pass in western hemisphere

[pmoleri]

Fix tests run to be able to run in US ET timezone without errors.

• First commit sets tests to run in America/New_York time zone.
  See results: https://github.com/IgniteUI/igniteui-angular/actions/runs/13441426761
  Note: TZ env var trick doesn't seem to work on Windows. Change PC timezone instead.

• Second commit fixes the tests by making sure that all dates are in local timezone.
  e.g.

  • new Date('2025-01-01') -> 2024-12-31 19:00 GMT -05:00 ❌ produces invalid results
  • ymd('2025-01-01') -> 2025-01-01 19:00 GMT -05:00 ✔

See also Angular's explanation here, which is what they use for their Date pipe.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them