Generator testing should assert cancellation token respect in a timely manner

[Youssef1313]

Per dotnet/runtime#68353, not respecting cancellation token has performance impact. The testing should assert that cancelling will happen in a timely manner.

[sharwell]

There is no straightforward way to test this on CI virtual machines with a wide variety of performance characteristics. In general, it seems analyzers are a better way to handle this.