I work as embedded developer in non-FAANG reputed company. I had Google onsite interview last year. I was amazed by the talent and the level of knowledge every engineer carried in problem solving. Unfortunately I couldn't crack some paradigms of problems. Then I determined a whole year to become fluent in algorithms. I have covered around 450 problems (267/166/19) overall in leetcode. I've really enjoyed the process. As an embedded engineer, I've have an edge on OS concepts, device drivers, networking. I even went through System design and Object Oriented design. I felt I'm ready for another interview with Google. But I had surprise at phone interview. The interviewer this time seems like very interested in testing.

Question:
The question was simple: "test calculator".

And suddenly he/she even asked me to use python unit test framework. I felt like I lost the game. I have never used it and I hardly emphasized on testing as I'm more interested on development roles. I tried to manage by writing some test code, test vector generator but I've no clear direction to tackle this scenario. My expectation was to face some algorithmic intensive question but this challenge seems like like a test-design question.

The question seemed connected to this video:

I got a call from recruiter in 2 days and I was notified about rejection as expected. But I asked recruiter about the way the interview was held. I wanted to understand if it was a testing role or development role (I was told it is generic software engineer). And recruited said it is advisory to have testing skills as well for interview. And now, I need to wait another year to get a interview chance with Google.

For an engineer interested in developer role, what is the level of expertise expected to have in testing design? Any suggestions for good material to revise testing design?

Please share similar experiences, comments, advice.