Question: Should you or should you not test single-line functions?

Answer: 
If they are part of your public interface then you should test them.
Some one-liners can be quite elegant/long.

I tend to not test "trivial" functions extensively, though there may be some tests (usually by inspection and one/two test cases) done just to make sure I've matched the documentation/specifications correctly (particularly if others may ever look at the doc and/or function).

Obviously, more complicated one-liners may require just as much testing as a "regular length" function (whatever regular length means to you).