When I go back to fix something or refactor old code, I know what an anxious experience it is to do that without test coverage, so having a basic test suite makes me happy.
Hobby projects? Less so, particularly if it's just me writing software. However, if I'm writing a particularly hairy piece of logic I'll absolutely write some tests for it (for example, I wrote a JS calendar plugin just to see if I could do better than the many existing ones, and I did all of the date logic myself using JS's terrible Date library).
They got divorced 1 yr after marriage.
So much for writing a test cases. Follow your intuition.
If I'm exploring an idea that I know is throwaway and won't be foundational for anything else, no.