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

Answer: 
Today it's one line, next week it's twenty lines.

Putting tests in place now warns you if you mistakenly break the behavior down the road.

Use your best judgment as to whether it's something that has any chance of changing in the future.
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).