Fix "Composing features" tutorial (#2698)

* Fix the filename

* Add `@MainActor` and `async` for testing the feature

---------

Co-authored-by: Stephen Celis <[email protected]>

Author: bricklife

Sources/ComposableArchitecture/Documentation.docc/Tutorials/MeetTheComposableArchitecture/01-Essentials/04-ComposingFeatures/01-04-02-code-0005.swift

@@ -1,8 +1,9 @@
 import ComposableArchitecture
 import XCTest
 
-class AppFeatureTests: XCTestCase {
-  func testIncrementInFirstTab() {
+@MainActor
+final class AppFeatureTests: XCTestCase {
+  func testIncrementInFirstTab() async {
 
   }
 }

Sources/ComposableArchitecture/Documentation.docc/Tutorials/MeetTheComposableArchitecture/01-Essentials/04-ComposingFeatures/01-04-02-code-0006.swift

@@ -1,8 +1,9 @@
 import ComposableArchitecture
 import XCTest
 
-class AppFeatureTests: XCTestCase {
-  func testIncrementInFirstTab() {
+@MainActor
+final class AppFeatureTests: XCTestCase {
+  func testIncrementInFirstTab() async {
     let store = TestStore(initialState: AppFeature.State()) {
       AppFeature()
     }

Sources/ComposableArchitecture/Documentation.docc/Tutorials/MeetTheComposableArchitecture/01-Essentials/04-ComposingFeatures/01-04-02-code-0007.swift

@@ -1,8 +1,9 @@
 import ComposableArchitecture
 import XCTest
 
-class AppFeatureTests: XCTestCase {
-  func testIncrementInFirstTab() {
+@MainActor
+final class AppFeatureTests: XCTestCase {
+  func testIncrementInFirstTab() async {
     let store = TestStore(initialState: AppFeature.State()) {
       AppFeature()
     }

Sources/ComposableArchitecture/Documentation.docc/Tutorials/MeetTheComposableArchitecture/01-Essentials/04-ComposingFeatures/01-04-02-code-0008.swift

@@ -1,8 +1,9 @@
 import ComposableArchitecture
 import XCTest
 
-class AppFeatureTests: XCTestCase {
-  func testIncrementInFirstTab() {
+@MainActor
+final class AppFeatureTests: XCTestCase {
+  func testIncrementInFirstTab() async {
     let store = TestStore(initialState: AppFeature.State()) {
       AppFeature()
     }

Sources/ComposableArchitecture/Documentation.docc/Tutorials/MeetTheComposableArchitecture/01-Essentials/04-ComposingFeatures/01-04-ComposingFeatures.tutorial

@@ -187,7 +187,7 @@
       @Step {
         Replace the two individual store properties with a single store of the `AppFeature` domain.
 
-        @Code(name: "AppFeatureTests.swift", file: 01-04-03-code-0001.swift, previousFile: 01-04-03-code-0001-previous.swift)
+        @Code(name: "AppFeature.swift", file: 01-04-03-code-0001.swift, previousFile: 01-04-03-code-0001-previous.swift)
       }
 
       @Step {
@@ -196,13 +196,13 @@
         domain. This is done by using key path syntax to single out the field of the state and
         the case of the action enum.
 
-        @Code(name: "AppFeatureTests.swift", file: 01-04-03-code-0002.swift)
+        @Code(name: "AppFeature.swift", file: 01-04-03-code-0002.swift)
       }
 
       @Step {
         And do the same for the second tab.
 
-        @Code(name: "AppFeatureTests.swift", file: 01-04-03-code-0003.swift)
+        @Code(name: "AppFeature.swift", file: 01-04-03-code-0003.swift)
       }
 
       That is all it takes. We now have a single ``ComposableArchitecture/Store`` powering this 
@@ -212,7 +212,7 @@
       @Step {
         Create a preview for `AppView` by initializing a store with the `AppFeature` domain.
 
-        @Code(name: "AppFeatureTests.swift", file: 01-04-03-code-0004.swift)
+        @Code(name: "AppFeature.swift", file: 01-04-03-code-0004.swift)
       }
 
       We can also run the feature in the simulator by updating the entry point of the app to be