unittests: Test tv_list_copy

Also found some bugs:

1. var_item_copy() always fails to copy v:_null_list and v:_null_dict. Fixing
   this should mean fixing `deepcopy(v:_null_list)` which should’ve been, but
   was not listed in #4615. This also fixes `deepcopy(v:_null_dict)`.
2. var_item_copy() crashes when trying to copy NULL string with `conv != NULL`.
3. `conv` argument is ignored when copying list unless `deep` is true, but it
   was not reflected in documentation.
4. `tv_dict_item_alloc_len()` allocated more memory then needed.
5. typvalt2lua was not able to handle self-referencing containers.

src/nvim/eval/typval.c

@@ -424,6 +424,7 @@ void tv_list_append_number(list_T *const l, const varnumber_T n)
 /// Make a copy of list
 ///
 /// @param[in]  conv  If non-NULL, then all internal strings will be converted.
+///                   Only used when `deep` is true.
 /// @param[in]  orig  Original list to copy.
 /// @param[in]  deep  If false, then shallow copy will be done.
 /// @param[in]  copyID  See var_item_copy().