Ignore parent class' method in unit testing with mockito

I have the following class which I try to test:

public class ClassA extends ClassB {
  ...
  public String methodToBeTested(){
    methodA1();
    methodB();
    return something;
  }
  ...
}

The methodB is parent class's method. I want to ignore this method or just stub it. I'm trying the following more and less:

public class TestClassA{
  @Mock
  ClassB instanceB;

  public String testTheMethodToBeTested{
    doNothing().when(instanceB).methodB();
    ClassA instanceA = new ClassA();
    String result = instanceA.methodToBeTested();
    String expected = "foo"
    assertTrue(result.equals(expected));
  }
  ...
}

However I see that methodB still is being executed. However in ClassA if I declare a classB variable and it's setter; and instead of methodB() write classB.methodB() my tests executes. But I am asked to not change the code being tested. Is there any way to test ClassA ?