Fix race condition in testHighPriorityTasksGetExecutedBeforeLowPriorityTasks

The assertion in this test was too strict. Even if the high priority tasks are scheduled first, they might finish after the low priority tasks (both high-priority and low-priority tasks sleep 100ms in these tests). For example the following is a valid recording of executions.

```
Set([high(0)])
Set([high(1), high(0)])
Set([high(1), high(2), high(0)])
Set([high(2), high(0), high(3), high(1)])
Set([high(2), high(3), high(1)])
Set([high(2), high(1)])
Set([high(2)])
Set([])
Set([high(4)])
Set([high(5), high(4)])
Set([high(6), high(5), high(4)])
Set([high(6), high(5), high(7), high(4)])
Set([high(6), high(5), high(7)])
Set([high(5), high(7)])
Set([high(5)])
Set([])
Set([high(8)])
Set([high(8), high(9)])
Set([high(8), high(9), low(0)])
Set([high(8), high(9), low(0), low(1)])
Set([high(8), high(9), low(0)])
Set([high(8), high(9)])
Set([high(9)])
```

Change the assertion to check that all high-priority tasks start executing before the first low-priority task starts executing.

rdar://145660858

Author: ahoppen

Tests/SemanticIndexTests/TaskSchedulerTests.swift

@@ -39,11 +39,15 @@ final class TaskSchedulerTests: XCTestCase {
         }
       },
       validate: { (recordings: [Set<TaskID>]) in
-        // Check that all high-priority tasks get executed before the low-priority tasks
-        let highPriorityRecordingSlice = recordings.dropLast(while: {
-          $0.isEmpty || $0.contains(where: \.isLowPriority)
+        // Check that all high-priority tasks start executing before the low-priority tasks
+        let highPriorityRecordingSlice = recordings.prefix(while: {
+          $0.isEmpty || $0.contains(where: \.isHighPriority)
         })
-        assertAllSatisfy(highPriorityRecordingSlice) { !$0.contains(where: \.isLowPriority) }
+        let taskIdsInHighPriorityRecordingSlice = Set(highPriorityRecordingSlice.flatMap { $0 })
+        XCTAssert(
+          taskIdsInHighPriorityRecordingSlice.isSuperset(of: (0..<10).map(TaskID.highPriority)),
+          "Low priority task started executing before high-priority task. Recording: \(recordings)"
+        )
 
         // Check that we never have more than the allowed number of low/high priority tasks, respectively
         assertAllSatisfy(recordings) { $0.count(where: \.isLowPriority) <= lowPriorityTasks }