Fix priority test

Priorities keep shifting since I first wrote this and are apparently
different on different OS's and different versions of the same OS.
Since the test is checking that entering and exiting tasks doesn't
effect the outer-scoped priority, I've set a variable based on the main
thread's priories, which should be inherited by the child.

Author: etcwilde

test/Concurrency/Runtime/async_task_priority_current.swift

@@ -17,10 +17,19 @@ extension TaskPriority: CustomStringConvertible {
   }
 }
 
+@available(SwiftStdlib 5.5, *)
+@main struct Main {
+  static func main() async {
+    print("main priority: \(Task.currentPriority)") // CHECK: main priority: TaskPriority(rawValue: [[#MAIN_PRIORITY:]])
+    await test_detach()
+    await test_multiple_lo_indirectly_escalated()
+  }
+}
+
 @available(SwiftStdlib 5.5, *)
 func test_detach() async {
   let a1 = Task.currentPriority
-  print("a1: \(a1)") // CHECK: TaskPriority(rawValue: 25)
+  print("a1: \(a1)") // CHECK: a1: TaskPriority(rawValue: [[#MAIN_PRIORITY]])
 
   // Note: remember to detach using a higher priority, otherwise a lower one
   // might be escalated by the get() and we could see `default` in the detached
@@ -30,8 +39,15 @@ func test_detach() async {
     print("a2: \(a2)") // CHECK: a2: TaskPriority(rawValue: 25)
   }.get()
 
-  let a3 = Task.currentPriority
-  print("a3: \(a3)") // CHECK: a3: TaskPriority(rawValue: 25)
+  await detach(priority: .default) {
+    let a3 = Task.currentPriority
+    // The priority of 'a3' may either be 21 (default) or elevated to that of
+    // the main function, whichever is greater.
+    print("a3: \(a3)") // CHECK: a3: TaskPriority(rawValue: [[#max(MAIN_PRIORITY,21)]]
+  }.get()
+
+  let a4 = Task.currentPriority
+  print("a4: \(a4)") // CHECK: a4: TaskPriority(rawValue: [[#MAIN_PRIORITY]])
 }
 
 @available(SwiftStdlib 5.5, *)
@@ -66,10 +82,4 @@ func test_multiple_lo_indirectly_escalated() async {
   print("default done") // CHECK: default done
 }
 
-@available(SwiftStdlib 5.5, *)
-@main struct Main {
-  static func main() async {
-    await test_detach()
-    await test_multiple_lo_indirectly_escalated()
-  }
-}
+