I am not sure if I am doing it right, but the tests indicate that the cancellation token should be passed from a given source.
We received the following error when we ran your code:
BankAccountTests.cs(158,23): warning xUnit1051: Calls to methods which accept CancellationToken should use TestContext.Current.CancellationToken to allow test cancellation to be more responsive. (xUnit1051 > xUnit.net)
I am sure the test case with the Tasks is the one with the issue, as indicated by the error message.