Add test for duplicate chunk names

[Zachary del Rosario]

We don't presently have a test (i.e. under ./tests/test_exercises.R) to check for duplicate chunk names. Presently I do this locally by (programmatically) running all exercises with NCmisc::list.functions.in.file() (see line), but that's a heavyweight solution.

Adding a test for duplicate chunk names (and 'runs without errors') would be useful enhancement!