* 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.
* Replace Prefire with a very similar but simpler and more direct approach
* Move PreviewTest snapshots to their new location (snapshotting default, mirrors UITests)
* Update acknowledgements
* Switch snapshot configuration extensions to a custom version
* Exclude snapshots from the PreviewTests target
* Fix acknowledgements
* Rename snapshotting preference keys
* Remove `skipPackagePluginValidation`
