Check the XCTest fallback handler *after* checking for other configurations. (#1542)

The new interop code orders the fallback handler _before_ the
`Configuration.all` check, meaning that issues recorded in detached
tasks _always_ fall back to XCTest. This can cause spurious test passage
because these issues get recorded as warnings by default. (Our own tests
are broken as a result).

Resolves rdar://170161483.

### 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.

---------

Co-authored-by: Jerry Chen <[email protected]>

Author: grynspan

Sources/Testing/Events/Event.swift

@@ -342,13 +342,18 @@ extension Event {
       if configuration.eventHandlingOptions.shouldHandleEvent(self) {
         configuration.handleEvent(self, in: context)
       }
-    } else if postToFallbackHandler(in: context) {
-      // The fallback event handler handled this event.
     } else {
       // The current task does NOT have an associated configuration. This event
       // will be lost! Post it to every registered event handler to avoid that.
-      for configuration in Configuration.all {
-        _post(in: context, configuration: configuration)
+      let configurations = Configuration.all
+      if configurations.isEmpty {
+        // There are no registered event handlers. Use the fallback event
+        // handler instead.
+        _ = postToFallbackHandler(in: context)
+      } else {
+        for configuration in configurations {
+          _post(in: context, configuration: configuration)
+        }
       }
     }
   }

Sources/Testing/Running/Runner.RuntimeState.swift

@@ -161,6 +161,24 @@ extension Configuration {
     }
   }
 
+#if DEBUG
+  /// Removes all configurations set in the current process.
+  ///
+  /// - Returns: The number of configuration instances that were removed from
+  ///   the current process' internal registry.
+  ///
+  /// - Warning: This function should never be called in the main testing
+  ///   process. Only use it in an exit test when you need a (relatively) clean
+  ///   slate for e.g. XCTest interop.
+  @discardableResult static func removeAll() -> Int {
+    Self._all.withLock { all in
+      let result = all.instances.count
+      all.instances.removeAll(keepingCapacity: false)
+      return result
+    }
+  }
+#endif
+
   /// An atomic counter that tracks the number of "current" configurations that
   /// have set ``EventHandlingOptions/isExpectationCheckedEventEnabled`` to
   /// `true`.

Tests/TestingTests/EventHandlingInteropTests.swift

@@ -47,6 +47,8 @@ struct EventHandlingInteropTests {
   /// its own fallback handler.
   @Test func `Post event without config -> fallback handler`() async throws {
     await #expect(processExitsWith: .success) {
+      Configuration.removeAll()
+
       let installer = try #require(Self.installer)
       try #require(
         installer(Self.capturingHandler), "Installation of fallback handler should succeed")