Use dynamic overlay for unimplemented stuff

Sources/CollectionView/CellFactory.swift

 import UIKit
+import XCTestDynamicOverlay
 
 public struct CellFactory<Model> {
   public struct Registrar {
@@ -65,10 +66,10 @@ extension CellFactory {
 
 #if DEBUG
 extension CellFactory {
-  public static func failing() -> CellFactory {
+  public static func unimplemented() -> CellFactory {
     CellFactory(
-      register: .init { _ in fatalError("Not implemented") },
-      build: .init { _, _, _ in fatalError("Not implemented") }
+      register: .init(register: XCTUnimplemented("\(Self.self).Registrar")),
+      build: .init(build: XCTUnimplemented("\(Self.self).Builder"))
     )
   }
 }

Sources/CollectionView/ViewConfigurator.swift

 import UIKit
+import XCTestDynamicOverlay
 
 public struct ViewConfigurator<View: UIView, Model> {
   public init(configure: @escaping (View, Model) -> Void) {
@@ -14,8 +15,8 @@ public struct ViewConfigurator<View: UIView, Model> {
 
 #if DEBUG
 extension ViewConfigurator {
-  public static func failing() -> ViewConfigurator {
-    ViewConfigurator { _, _ in fatalError("Not implemented") }
+  public static func unimplemented() -> ViewConfigurator {
+    ViewConfigurator(configure: XCTUnimplemented("\(Self.self)"))
   }
 }
 #endif