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.

UnitTests/SupportingFiles/target.yml

@@ -46,6 +46,9 @@ targets:
     sources:
     - path: ../Sources
     - path: ../SupportingFiles
+    - path: ../../DevelopmentAssets
+    - path: ../../ElementX/Sources/Other/Extensions/Publisher.swift
+    - path: ../../ElementX/Sources/Other/Extensions/XCTestCase.swift
     - path: ../../ElementX/Sources/Other/InfoPlistReader.swift
     - path: ../../Tools/Scripts/Templates/SimpleScreenExample/Tests/Unit
-    - path: ../../DevelopmentAssets
+