There are several possible reasons why you didn't include any tests in a take home exercise, and they are all bad.
1) You ran out of time to write tests.
This might be the least bad reason. You can probably talk your way out of this by explaining what tests you would have written if you had the time.
2) You don't know how to write tests.
This shows that you have only worked at places the don't do testing and that you haven't taken the initiative to learn about testing on your own.
3) You know how to write tests, but you don't think they are needed.
You could probably tell the interviewer why tests were not important for the particular exercise. But I have never seen a take home exercise that doesn't require a basic algorithm and some tricky conditional business logic. Both of which would benefit from unit tests.
4) The code you wrote isn't testable.
This is probably the worst reason. Everything is in one or two giant functions and you can't inject dependencies for testing without refactoring everything. This will frighten your interviewer.
Ego-depletion is mentioned, not always by name, in hundreds productivity and focus related articles and books published in the last 5 years or so.
Steve Jobs is often used as an example because he wore the same outfit to work every day, purportedly so he would not have to waste any crucial willpower early in the day on such a trivial task as choosing what to wear.
Does anyone know what the fish oil additive is used for?
I was recently researching how Trader Joe's coffee is so inexpensive and found some reviews noting a "fish sauce" flavor and a "fishy" flavor in two TJs coffee products.
The problem is that many interviewers don't know how to suss out a good engineer. So they just ask questions that Google and Amazon ask. And that's why you spend half of your interview pre-optimizing contrived algorithms down to O(n) when the probability of the startup ever running code at scale is very small.
I suspect Warren Buffet may have pushed for Stumpf's resignation/termination, as well as other damage control measures. This is a similar situation to the Solomon scandal, although Buffet has a smaller stake in WFC.