Fix unittest for std.array.assocArray

This pull request includes the following changes.

* Split the unittest into a documented unittest and a normal unittest.
* Use documented unittest instead of a normal comment and unittest.
  Basically I use the example in the comment but I leave the line `assert(is(typeof(a) == string[int]));` in the original unittest.
* Mark one unittest as safe, pure and nothrow.
* Mark another unittest as pure. It cannot be marked as safe and nothrow because of `std.range.zip`.