eval/typval,tests: Fix extending list with itself, add tests

Adds unit test for tv_list_extend and regression test for extend() VimL
function.

test/unit/eval/helpers.lua

@@ -439,4 +439,6 @@ return {
 
   list_items=list_items,
   dict_items=dict_items,
+
+  empty_list = {[type_key]=list_type},
 }