Fix OperationQueue scheduling logic to prevent disruption of operation lists

I believe the original intention was to adust `__nextPriorityOperation`.
Doing so for `__nextOperation` has no sense and leads to crosslinking
of the main operation list and corresponding priority operation list.
OperationQueue in such state would crash or hang while attempting
to schedule subsequent operation.

Author: lxbndr

Sources/Foundation/Operation.swift

@@ -993,7 +993,7 @@ open class OperationQueue : NSObject, ProgressReporting {
                 // if the cached state is possibly not valid then the isReady value needs to be re-updated
                 if Operation.__NSOperationState.enqueued == operation._state && operation._fetchCachedIsReady(&retest) {
                     if let previous = prev?.takeUnretainedValue() {
-                        previous.__nextOperation = next
+                        previous.__nextPriorityOperation = next
                     } else {
                         _setFirstPriorityOperation(prio, next)
                     }

Tests/Foundation/Tests/TestOperationQueue.swift

@@ -34,6 +34,7 @@ class TestOperationQueue : XCTestCase {
             ("test_CancelWhileSuspended", test_CancelWhileSuspended),
             ("test_OperationOrder", test_OperationOrder),
             ("test_OperationOrder2", test_OperationOrder2),
+            ("test_ExecutionOrder", test_ExecutionOrder),
             ("test_WaitUntilFinished", test_WaitUntilFinished),
             ("test_OperationWaitUntilFinished", test_OperationWaitUntilFinished),
             ("test_CustomOperationReady", test_CustomOperationReady),
@@ -531,6 +532,32 @@ class TestOperationQueue : XCTestCase {
         XCTAssertEqual(array, [5, 4, 3, 2, 1])
     }
 
+    func test_ExecutionOrder() {
+        let queue = OperationQueue()
+        
+        let didRunOp1 = expectation(description: "Did run first operation")
+        let didRunOp1Dependency = expectation(description: "Did run first operation dependency")
+        let didRunOp2 = expectation(description: "Did run second operation")
+        var didRunOp1DependencyFirst = false
+        
+        let op1 = BlockOperation {
+            didRunOp1.fulfill()
+            XCTAssertTrue(didRunOp1DependencyFirst, "Dependency should be executed first")
+        }
+        let op1Dependency = BlockOperation {
+            didRunOp1Dependency.fulfill()
+            didRunOp1DependencyFirst = true
+        }
+        op1.addDependency(op1Dependency)
+        queue.addOperations([op1, op1Dependency], waitUntilFinished: false)
+        
+        queue.addOperation {
+            didRunOp2.fulfill()
+        }
+        
+        waitForExpectations(timeout: 1.0)
+    }
+
     func test_WaitUntilFinished() {
         let queue1 = OperationQueue()
         let queue2 = OperationQueue()