Run runActivity on main thread (#895)

[sk409]

Fixes #895 .

[mbrandonw]

Hi @sk409, I believe the only way runActivity could be called on a background thread is if your test is running on a background thread. Instead of this change can you mark your test with @MainActor?

We have plans to more deeply integrate snapshot testing with Swift's concurrency tools, but until then you will need to be responsible for calling its APIs on the main thread.

[sk409]

@mbrandonw
My app uses SDWebImageSwiftUI to display images.
SDWebImageSwiftUI updates views on the main thread, so when I run the test on the main thread, XCTWaiter.wait blocks updating views. Therefore, the image will no longer be displayed in the snapshot.
For the reasons mentioned above I need to run my tests on a background thread instead of the main thread.

※SDWebImageSwiftUI performs the following processing on the main thread.

• https://github.com/SDWebImage/SDWebImageSwiftUI/blob/master/SDWebImageSwiftUI/Classes/ImageManager.swift#L99
• https://github.com/SDWebImage/SDWebImageSwiftUI/blob/master/SDWebImageSwiftUI/Classes/ImageManager.swift#L20