Fix a regression that prevents saving attachments. (#1400)

This PR ensures that the attachment-saving logic in `Runner` is applied
consistently. I made the mistake of moving that logic to
`configureEventHandlerRuntimeState()` because I'd forgotten that that
function only has an effect when we're running our own tests. And, as a
result, all our tests continued to pass because the code was in place
for our tests (and for nobody else.)

I've moved the code to a different location that we will always call
when running a `Runner` instance, so the issue is resolved now. I don't
have a great way to set up a unit test for this; tested manually at
desk.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Attachments/Attachment.swift

@@ -501,6 +501,26 @@ extension Attachment where AttachableValue: ~Copyable {
   }
 }
 
+extension Runner {
+  /// Modify this runner's configured event handler so that it handles "value
+  /// attached" events and saves attachments where necessary.
+  mutating func configureAttachmentHandling() {
+    configuration.eventHandler = { [oldEventHandler = configuration.eventHandler] event, context in
+#if !SWT_NO_FILE_IO
+      var event = copy event
+      if case .valueAttached = event.kind {
+        guard let configuration = context.configuration,
+              configuration.handleValueAttachedEvent(&event, in: context) else {
+          // The attachment could not be handled, so suppress this event.
+          return
+        }
+      }
+      oldEventHandler(event, context)
+#endif
+    }
+  }
+}
+
 extension Configuration {
   /// Handle the given "value attached" event.
   ///
@@ -517,7 +537,7 @@ extension Configuration {
   /// not need to call it elsewhere. It automatically saves the attachment
   /// associated with `event` and modifies `event` to include the path where the
   /// attachment was saved.
-  func handleValueAttachedEvent(_ event: inout Event, in eventContext: borrowing Event.Context) -> Bool {
+  fileprivate func handleValueAttachedEvent(_ event: inout Event, in eventContext: borrowing Event.Context) -> Bool {
     guard let attachmentsPath else {
       // If there is no path to which attachments should be written, there's
       // nothing to do here. The event handler may still want to handle it.

Sources/Testing/Running/Runner.RuntimeState.swift

@@ -48,16 +48,6 @@ extension Runner {
     }
 
     configuration.eventHandler = { [oldEventHandler = configuration.eventHandler] event, context in
-#if !SWT_NO_FILE_IO
-      var event = copy event
-      if case .valueAttached = event.kind {
-        guard let configuration = context.configuration,
-              configuration.handleValueAttachedEvent(&event, in: context) else {
-          // The attachment could not be handled, so suppress this event.
-          return
-        }
-      }
-#endif
       RuntimeState.$current.withValue(existingRuntimeState) {
         oldEventHandler(event, context)
       }

Sources/Testing/Running/Runner.swift

@@ -416,6 +416,7 @@ extension Runner {
   private static func _run(_ runner: Self) async {
     var runner = runner
     runner.configureEventHandlerRuntimeState()
+    runner.configureAttachmentHandling()
 
     // Track whether or not any issues were recorded across the entire run.
     let issueRecorded = Locked(rawValue: false)