[Test][Concurrency] Fix reflect_task test to avoid a race condition.

This isn't a nice fix, because I can't see a good way to fix this problem
robustly.  The issue is that the Concurrency runtime may mutate the AsyncTask
structure from a thread other than the one that is reflecting the current
task, so looking through at the child tasks is inherently dangerous and
may cause the test to crash at random, even though it will *often* work.

To make this a lot less likely, add a .25s delay before reflecting the
parent task.

rdar://150478597

Author: al45tair

test/Concurrency/Reflection/reflect_task.swift

@@ -63,11 +63,35 @@ func testNestedCallsTask() async {
   // CHECK:     Chunk at {{0x[0-9a-fA-F]*}} length {{[1-9[[0-9]*}} kind {{[0-9]*}}
 }
 
+// ###HACK: Avoid a race condition by waiting a bit.
+//
+// If we reflect the current task immediately, there is a chance that the
+// runtime is busy manipulating the task structure (e.g. allocating using the
+// task allocator on a child task, or adding a child task) in parallel with
+// the reflection.  This can cause this test to crash when the
+// swift-reflection-test binary asks to read from an uninitialized address.
+//
+// While we could stop the crashing by changing the pipe memory reader to
+// return error codes when it is unable to read memory, that would still
+// result in this test failing randomly.  It's not obvious what the "proper"
+// fix for this should be; the Concurrency runtime isn't going to export
+// anything that would help here.
+func dodgeRaceCondition() async {
+  if #available(SwiftStdlib 5.7, *) {
+    try! await Task.sleep(for: .milliseconds(250))
+  } else {
+    fatalError("This test shouldn't be running against old stdlibs.")
+  }
+}
+
 func testOneAsyncLet() async {
   reflectionLog(str: "testOneAsyncLet")
   // CHECK: testOneAsyncLet
 
   async let alet = sleepForever()
+
+  await dodgeRaceCondition()
+
   reflect(asyncTask: _getCurrentTaskShim())
   // CHECK: Async task {{0x[0-9a-fA-F]*}}
   // CHECK: children = {
@@ -81,6 +105,9 @@ func testMultipleAsyncLet() async {
 
   async let alet1 = sleepForever()
   async let alet2 = sleepForever()
+
+  await dodgeRaceCondition()
+
   reflect(asyncTask: _getCurrentTaskShim())
   // CHECK: Async task {{0x[0-9a-fA-F]*}}
   // CHECK: children = {