Relax the assertion in testHighPriorityTasksGetExecutedBeforeLowPriorityTasks

I just received the following recordings during local test execution:

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

These recordings didn't satisfy the old assertion because a low-priority task started executing before the last high-priority task. But really what happened is that `low(0)`, `high(8)`, `low(1)`, and `high(9)` were all scheduled simultaneously and `low(0)` just happened to start executing a tiny bit before `high(8)`. To account for these races, relax the assertion to check that all high-priority tasks start executing before the first low-priority task finishes executing.

Author: ahoppen

Tests/SemanticIndexTests/TaskSchedulerTests.swift

@@ -39,9 +39,14 @@ final class TaskSchedulerTests: XCTestCase {
         }
       },
       validate: { (recordings: [Set<TaskID>]) in
-        // Check that all high-priority tasks start executing before the low-priority tasks
-        let highPriorityRecordingSlice = recordings.prefix(while: {
-          $0.isEmpty || $0.contains(where: \.isHighPriority)
+        // Check that all high-priority tasks start executing before the first low-priority task finishes
+        var startedLowPriorityTasks: Set<TaskID> = []
+        let highPriorityRecordingSlice = recordings.prefix(while: { recording in
+          if startedLowPriorityTasks.contains(where: { !recording.contains($0) }) {
+            return false
+          }
+          startedLowPriorityTasks.formUnion(recording.filter(\.isLowPriority))
+          return true
         })
         let taskIdsInHighPriorityRecordingSlice = Set(highPriorityRecordingSlice.flatMap { $0 })
         XCTAssert(