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/Other/Extensions/Publisher.swift

@@ -36,3 +36,17 @@ extension Publisher where Output == String, Failure == Never {
         }
     }
 }
+
+extension Published.Publisher {
+    /// Returns the next output from the publisher skipping the current value stored into it (which is readable from the @Published property itself).
+    /// - Returns: the next output from the publisher
+    var nextValue: Output? {
+        get async {
+            var iterator = values.makeAsyncIterator()
+            
+            // skips the publisher's current value
+            _ = await iterator.next()
+            return await iterator.next()
+        }
+    }
+}