Tests for comments

Delimited comments (<# ... #>) are not implemented yet, so the tests are
marked Explicit or are commented out.

I made the nit-picky test about comment tokenization a `[Combinatorial]` test, later to be extended with delimited comments, so for the time being there are a number of possible values for both arguments commented out. The question would be whether that's okay or whether they should not be commented and instead the whole test marked as `[Explicit]` until we have delimited comments, real literals and type literals.