Add some imports required to compile Keystone for testing (#24617)

* Configure the Keystone scheme to run tests

* Add some `import`s required to compile Keystone for testing

Unfortunately, there are many imports that we need to add.

I am using a combination of brute force "build, see what failed, fix,
repeat" and of strategic grepping like

```
grep -rlE '\bBlog\b' WordPress/Classes Sources \
  --exclude-dir=Sources/WordPressData -I --null \
  | xargs -0 grep -L 'WordPressData'
```

To identify usages of WordPressData types that don't have the required
import.

Interestingly, this exercise is revealing additional missing imports,
such as for WordPressKit.

Author: mokagio

Tests/KeystoneTests/Helpers/AccountBuilder.swift

@@ -1,4 +1,5 @@
 import Foundation
+import WordPressData
 
 @testable import WordPress
 

Tests/KeystoneTests/Helpers/BlogBuilder.swift

@@ -1,5 +1,7 @@
+import CoreData
 import Foundation
 import XCTest
+import WordPressData
 
 @testable import WordPress
 

Tests/KeystoneTests/Helpers/ModelTestHelper.swift

@@ -1,5 +1,6 @@
 import Foundation
 import CoreData
+import WordPressData
 @testable import WordPress
 
 public class ModelTestHelper: NSObject {

Tests/KeystoneTests/Helpers/PageBuilder.swift

@@ -1,3 +1,4 @@
+import CoreData
 import Foundation
 
 @testable import WordPress

Tests/KeystoneTests/Helpers/PostBuilder.swift

@@ -1,4 +1,5 @@
 import Foundation
+import WordPressData
 
 @testable import WordPress
 

Tests/KeystoneTests/Helpers/ReaderPostBuilder.swift

@@ -1,3 +1,4 @@
+import WordPressData
 @testable import WordPress
 
 /// Builds a `ReaderPost`

Tests/KeystoneTests/Helpers/SiteSuggestionsServiceMock.swift

@@ -1,3 +1,4 @@
+import CoreData
 import Foundation
 
 @testable import WordPress

Tests/KeystoneTests/Tests/Features/Media/StockPhotos/MockStockPhotosService.swift

@@ -1,3 +1,4 @@
+import Foundation
 @testable import WordPress
 
 final class MockStockPhotosService: StockPhotosService {

Tests/KeystoneTests/Tests/Features/Notifications/ApproveCommentActionTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+import WordPressData
 @testable import WordPress
 @testable import FormattableContentKit
 

Tests/KeystoneTests/Tests/Features/Notifications/FormattableCommentContentTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+import WordPressData
 @testable import WordPress
 @testable import FormattableContentKit