Add preferences test

Tests/TokamakReconcilerTests/PreferenceTests.swift

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+// Copyright 2021 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 6/29/22.
+//
+
+import XCTest
+
+@_spi(TokamakCore) @testable import TokamakCore
+import TokamakTestRenderer
+
+private enum TestKey: PreferenceKey {
+  static let defaultValue: Int = 0
+  static func reduce(value: inout Int, nextValue: () -> Int) {
+    value += nextValue()
+  }
+}
+
+final class PreferenceTests: XCTestCase {
+  func testPreferenceAction() {
+    struct TestView: View {
+      public var body: some View {
+        Text("")
+          .preference(key: TestKey.self, value: 2)
+          .preference(key: TestKey.self, value: 3)
+          .onPreferenceChange(TestKey.self) {
+            XCTAssertEqual($0, 5)
+          }
+      }
+    }
+    let reconciler = TestFiberRenderer(.root, size: .init(width: 500, height: 500))
+      .render(TestView())
+    reconciler.fiberChanged(reconciler.current)
+  }
+
+  func testOverlay() {
+    struct TestView: View {
+      var body: some View {
+        Text("")
+          .preference(key: TestKey.self, value: 2)
+          .preference(key: TestKey.self, value: 3)
+          .overlayPreferenceValue(TestKey.self) {
+            Text("\($0)")
+          }
+      }
+    }
+
+    let reconciler = TestFiberRenderer(.root, size: .init(width: 500, height: 500))
+      .render(TestView())
+    var overlayValue: FiberReconciler<TestFiberRenderer>.Fiber? {
+      reconciler.current // RootView
+        .child? // LayoutView
+        .child? // ModifiedContent
+        .child? // _ViewModifier_Content
+        .child? // TestView
+        .child? // _DelayedPreferenceView
+        .child? // ModifiedContent
+        .child? // _OverlayLayout
+        .child?.sibling? // _PreferenceReadingView
+        .child
+    }
+
+    reconciler.expect(overlayValue, equals: Text("5"))
+  }
+}