Rework availability for executor to avoid warnings

Sources/AsyncSequenceValidation/Job.swift

@@ -19,6 +19,6 @@ struct Job: Hashable, @unchecked Sendable {
   }
 
   func execute() {
-    _swiftJobRun(unsafeBitCast(job, to: UnownedJob.self), AsyncSequenceValidationDiagram.Context.executor.asUnownedSerialExecutor())
+    _swiftJobRun(unsafeBitCast(job, to: UnownedJob.self), AsyncSequenceValidationDiagram.Context.unownedExecutor)
   }
 }

Sources/AsyncSequenceValidation/Test.swift

@@ -67,25 +67,62 @@ extension AsyncSequenceValidationDiagram {
   }
 
   struct Context {
+#if swift(<5.9)
     final class ClockExecutor: SerialExecutor {
-      @available(macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0, *)
+      func enqueue(_ job: UnownedJob) {
+        job._runSynchronously(on: self.asUnownedSerialExecutor())
+      }
+
+      func asUnownedSerialExecutor() -> UnownedSerialExecutor {
+        UnownedSerialExecutor(ordinary: self)
+      }
+    }
+
+    private static let _executor = ClockExecutor()
+
+    static var unownedExecutor: UnownedSerialExecutor {
+      _executor.asUnownedSerialExecutor()
+    }
+#else
+    @available(macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0, *)
+    final class ClockExecutor_5_9:  SerialExecutor {
       func enqueue(_ job: __owned ExecutorJob) {
         job.runSynchronously(on: asUnownedSerialExecutor())
       }
 
-      @available(*, deprecated) // known deprecation warning
+      func asUnownedSerialExecutor() -> UnownedSerialExecutor {
+        UnownedSerialExecutor(ordinary: self)
+      }
+    }
+
+    final class ClockExecutor_Pre5_9: SerialExecutor {
+      @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+      @available(*, deprecated, message: "Implement 'enqueue(_: __owned ExecutorJob)' instead")
       func enqueue(_ job: UnownedJob) {
-        job._runSynchronously(on: asUnownedSerialExecutor())
+        job._runSynchronously(on: self.asUnownedSerialExecutor())
       }
-      
+
       func asUnownedSerialExecutor() -> UnownedSerialExecutor {
         UnownedSerialExecutor(ordinary: self)
       }
     }
 
+    private static let _executor: AnyObject = {
+      if #available(macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0, *) {
+        return ClockExecutor_5_9()
+      } else {
+        return ClockExecutor_Pre5_9()
+      }
+    }()
+
+    static var unownedExecutor: UnownedSerialExecutor {
+      (_executor as! any SerialExecutor).asUnownedSerialExecutor()
+    }
+#endif
+
     static var clock: Clock?
 
-    static let executor = ClockExecutor()
+
 
     static var driver: TaskDriver?