Retrofit deferFulfillment onto snapshot tests. (#3641)

* Retrofit `deferFulfillment` onto snapshot tests.

* Convert a bunch of preview tests to the new fulfillment publisher

* Convert more tests

* Remove unneeded delays from the remaining tests

* Remove snapshotting delay option.

ElementX/Sources/Screens/EmojiPickerScreen/View/EmojiPickerScreen.swift

@@ -97,7 +97,9 @@ struct EmojiPickerScreen_Previews: PreviewProvider, TestablePreview {
     static var previews: some View {
         EmojiPickerScreen(context: viewModel.context, selectedEmojis: ["😀", "😄"])
             .previewDisplayName("Screen")
-            .snapshotPreferences(delay: 0.5)
+            .snapshotPreferences(expect: viewModel.context.$viewState.map { state in
+                !state.categories.isEmpty
+            })
     }
 }