如何在Golang中的相同结构方法中模拟结构方法？

I have troubles with mocking.. I have same code:

type OtherInterface interface {
   Do() error
}
type MyInterface interface {
   MethodA() error
   MethodB() error
}
type MyStruct struct {
  OtherStruct OtherInterface
}
func(s *MyStruct) MethodA() error {
  return s.MethodB()
}
func(s *MyStruct) MethodB() error {
  return s.OtherStruct.Do()
}

And test code of MethodB:

type MockOtherStruct struct {
  Do_Error error
}
func (t *MockOtherStruct) Do() error {return t.Do_Error}


TestMyStruct_MethodB(t *testing.B) {
  expectedError = nil
  mos := &MockOtherStruct{
    Do_Error: nil
  }
  ms := MyStruct{
   OtherStruct: mos,
  }
  err := ms.MethodB()
  if err != expectedError {
    t.Fatal(err)
  }      
}

But how can I test MethodA?

Yes, I can also mock OtherStruct and use not mocked MethodB, but in big structs I can have many fields with other structs, but use one internal method..