Change default theme jest config platform from iOS -> win32

[lyzhan7]

Platforms Impacted

• iOS
• macOS
• win32 (Office)
• windows
• android

Description of changes

This PR is related to #2481, decided to make this a separate PR in order to keep the original as small as possible.

Why this change is required for PR 2481:

• Currently the default-theme tests run on iOS, and before PR 2481 goes in iOS will have been using web/windows alias tokens. So technically, the default-theme tests were never really testing iOS, they've been generally testing web/windows theme creation. Also, the default-theme tests include tests for high contrast mode and 'dynamic' mode - neither of these exist on iOS yet.
• After PR 2481 iOS will be using iOS alias tokens. This means the default-theme tests will be using iOS alias tokens. It doesn't make sense to be running high contrast tests on iOS, so updating these tests to actually be running on a platform that has high contrast
• Note - these tests still do run on iOS, I had to make a change in PR 2481 so that on iOS, if we try to access high contrast mode, instead of throwing an error we return the light mode tokens. I could keep the platform here set to iOS, just the high contrast mode tests will be really testing light mode tokens

For more context behind the current state of the default theme see #2492

Verification

No visual changes

Pull request checklist

This PR has considered (when applicable):

• Automated Tests
• Documentation and examples
• Keyboard Accessibility
• Voiceover
• Internationalization and Right-to-left Layouts

[lyzhan7]

I realized another option is to change the platform to windows instead of win32, maybe this would make more sense since the default-theme was created with with web/windows tokens in mind? Not sure what the difference is between web/windows and win32 tokens. Thinking I'll leave it as is for now though since win32 is used more often & might be better to have more test coverage on win32