Add unit test coverage for simple selection keywords.

"molindex" and "atomtype" keywords are now covered by separate tests.
Also updated one test to use the "z" keyword to have that covered.
Added a comment in sm_simple.cpp about unreachable code that is not
currently covered by tests.

Part of #651.

Change-Id: Id9ac9ea8e7d144214537265d9ed89335684934f1