Congratulations for getting into TDD, and for putting an effort to write a blog post about it :)!
Correct me if I am wrong, but looking at your code, it seems like your tests are not actually testing the Calculator React component, but instead testing another piece of code that is completely independent from the Calculator React component.
To be more precise, your tests are testing the Calc class that you have in .test.js file. That Calc class is local for .test.js file and has nothing to do with Calculator React component, meaning you are not testing your React component, you are testing the Calc class that is not really used anywhere besides in tests.
It seems to me like your way of thinking was: I will first write this method in Calc, test it, and if it works, I will write similar/same code in the Calculator component. However, that is not what automated testing is about. You want your tests to directly test real code, your Calculator component. So for example if you make a mistake in Calculator or remove a method, tests should fail and indicate that (which will not happen at the moment). Tests are supposed to tell you at any moment if your application is doing what is should be doing, with no assumptions needed from your side (I am referring to your assumption that code in Calc is similar/equal to code in Calculator).
So, what you want to do is import Calculator.js into your Calculator.test.js and then write tests testing the component. Check tutorials on how to write tests for React components.
Additionally, if you decide to extract piece of arithmetic logic from the Calculator.js into a class that is for example called Calc (not a great name but ok) and stored in Calc.js, and you are importing Calc.js into Calculator.js and using it there, you could write separate Calc.test.js to test that piece of logic that is not React specific. Those tests would be similar to what you wrote in this post. However, you would still want to keep Calculator.test.js and have some tests there, to test the component itself.
