Good list of how you interview/test and what your expectations are! I'd have to admit, I don't often even consider writing a test for a method that would just return a join() - I trust the join() method in the underlying language 'just works', although there are other reasons for writing tests (error handling, etc).

I do some PHP teaching, and one thing I got some good feedback from was giving students assignments that had asserts in them. Not quite unit-testing specifically (not using a full testing harness), but I'd give them some shell code with assert statements and they'd need to 'make it work'.

The sharp ones actually made it work, and gave me feedback that they appreciated that a lot, as it showed them how to think about breaking down things in to testable parts (even trivial stuff). The not-so-sharp ones... easier to spot when they'd give me back code that obviously was never even run in the first place.