Enable `CupertinoPopupSurface`'s backdrop filter test on skwasm once the Skia bug is fixed

https://github.com/flutter/flutter/pull/151430 added a backdrop filter to `CupertinoPopupSurface`, but the filter does not work on skwasm due to a Skia bug https://github.com/flutter/flutter/issues/152026. Once the issue is closed, we should unskip these tests.