Need to initialize os_unfair_lock guarding the fallback event handler (#1478)

Prior to this fix, we didn't have an explicit init which could lead to
garbage values.

Resolves rdar://167861118

### 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: jerryjrchen

Sources/_TestingInterop/FallbackEventHandler.swift

@@ -22,7 +22,9 @@ private nonisolated(unsafe) let _fallbackEventHandler = {
     minimumCapacity: 1,
     makingHeaderWith: { _ in nil }
   )
-  result.withUnsafeMutablePointerToHeader { $0.initialize(to: nil) }
+  result.withUnsafeMutablePointerToElements { lock in
+    lock.initialize(to: .init())
+  }
   return result
 }()
 #else