1) For the love of Edsger Dijkstra, TDD your coding exercise. TDD if it is a homework assignment. TDD if it is live coding. If the interviewers are not visibly happy that you did TDD, then that is a sign that they don't care about testing in their job either. Once you have even a little bit of practice it starts to become easy. Just do it already.
2) Create a shell REPL that takes user input and then outputs things and some tests that exercise the inputs and outputs. This is the hardest part of TDD'ing a coding exercise and is completely independent of the actual logic of any coding exercise, so this will pay dividends in any exercise of your primary language you are looking at.
3) For the love of Ada Lovelace, use git from the start of your homework exercise. Don't just send a file. Don't just make one commit when you finish. This will help the interviewers see your process and help you remember how you made the thing 3 weeks later.