Don't expect anything about the base constructor. For your code, I would make sure that Car
is a vehicle and then check that the default wheelNumber was set properly (and this only if there were a public method for knowing that).
If the constructor for Car
repeated the code in Vehicle
involving the wheelNumber
your test should still pass. And then when someone refactors the code so that it is done properly the test should still pass. That way you know that you haven't changed your functionality.