`TestSourceKitLSPClient.handle` created a new `Task`. This means that we could swap the order of notifications received from SourceKit-LSP. In most cases this doesn’t matter but `BackgroundIndexingTests.testProduceIndexLogWithTaskID` checks that the notification to start a structured log is received before the first report, which might not be the case because of the `Task` here.
Change `PendingNotifications` to a class with a `ThreadSafeBox` to remove the need for a `Task`.
rdar://147814254
a151ca413c