Test Visibility of ActivityIndicator

[johnzhou721]

Closes #2838 which was inactive for a few months -- tell me if this is too short of a time to assume non-working-on-ness

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[johnzhou721]

Ready for review. Let me know if you want things to keep track of visibility on macOS by subclassing the native class to ensure that start() and stop() are called in the code... but then you'd risk not calling super().start() and super().stop() in your subclass... and the test will still pass... so since there's no native API for visibliity on macOS, there's pretty much no way to test that part.

[freakboy3742]

A couple of minor cleanups to code comments, but otherwise this looks good.

I'm not overly concerned about the macOS tests. This test is more about ensuring platforms that require an explicit hide are implementing it correctly; on macOS and GTK, the widget 'does the right thing' out of the box, so additional steps would be overkill.

[johnzhou721]

This test is more about ensuring platforms that require an explicit hide are implementing it correctly;

Thanks for the review. Yes, this is mainly following up on the Winforms PR since I made logic issues there.