Writing unittests for abstract classes

Consider this example:

class A:
    def do_stuff(self):
        # ...
        some_var = self.helper_method()
        # ...

    def helper_method(self):
        # This method must be implemented by subclass
        raise NotImplementedError()

class B(A):
    def helper_method(self):
        # implementation for class B

class C(A):
    def helper_method(self):
        # implementation for class C

My task is to write unit tests for A, B and C classes (especially for do_stuff).

But how can I test A class if I cannot use some of it methods directly? Should I just test only B and C classes (which have implementation for helper_method) or maybe there is common way for testing abstract classes in Python?