Improve StartWithContinuations test in AsyncType tests

[majocha]

Use a local cancellation token instead of Async.CancelDefaultToken() to cancel just one computation.

When running tests in parallel I've seen OperationCancelledException raised in unrelated tests of the same assembly.
It seems despite the fact this test collection has DisableParallelization = true, xUnit sometimes can't perfectly isolate the test cases.

[github-actions]

✅ No release notes required

[majocha]

New version goes like this:

module AsyncType =

    type ExpectedContinuation = Success | Exception | Cancellation

    [<Fact>]
    let startWithContinuations() =

        let cont actual expected _ =
            if expected <> actual then
                failwith $"expected {expected} continuation, but ran {actual}"

        let onSuccess = cont Success
        let onExeption = cont Exception
        let onCancellation = cont Cancellation

        let expect expected cancellationToken computation =
            Async.StartWithContinuations(computation, onSuccess expected, onExeption expected, onCancellation expected, ?cancellationToken = cancellationToken)

        let cancelledToken =
            let cts = new CancellationTokenSource()
            cts.Cancel()
            Some cts.Token

        async { return () } |> expect Cancellation cancelledToken

        async { failwith "computation failed" } |> expect Exception None

        async { return () } |> expect Success None