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
```swift
// Poll until the `CancelRequestNotification` has been propagated to the request handling.
for _ in 0..<Int(defaultTimeout * 100) {
if Task.isCancelled {
break
}
usleep(10_000)
}
```
I saw a few non-deterministic test failures. I think the issue was that handling of the `CancelRequestNotification` is done asynchronously, which left a short window in which the sourcekitd diagnostics request could run and return results instead of being cancelled. Wait for the diagnostic request to actually be cancelled before running it for real.
While doing this, also introduce proper sourcekitd test hooks instead of relying on the preparation test hooks, which just got run as a side effect.
I saw a failure in CI where a sourcekitd diagnostic request got cancelled without an LSP cancellation request. I am suspecting that there is some implicit cancellation going on in sourcekitd despite `key.cancel_builds: 0`, which doesn’t make sense to me. Adding some explicit logging to sourcekit-lsp to check if we cancel the request for some reason.
