Remove sourcekitd test hooks

[ahoppen]

Turns out that sourcekitd test hooks were a bad idea because of the following comment that I wrote:

`testHooks` are only considered when an instance is being created. If a sourcekitd instance at the given path already exists, its test hooks will be used.

During test execution in Xcode, we generate a bunch of SourceKitServer instances in the same process that all call DynamicallyLoadedSourceKitD.getOrCreate. Now, if testDontReturnEmptyDiagnosticsIfDiagnosticRequestIsCancelled is not the first test being executed in the process (which usually it is not), the test hooks in it won’t get used.

Switch back to using the preparation hooks, essentially reverting https://github.com/apple/sourcekit-lsp/pull/1412 and keeping the following snippet to fix the underlying issue

// Poll until the `CancelRequestNotification` has been propagated to the request handling.
for _ in 0..<Int(defaultTimeout * 100) {
  if Task.isCancelled {
    break
  }
  usleep(10_000)
}

[ahoppen]

@swift-ci Please test