unittests: Add tests for tv_list_find*() functions

Additional modifications:

- More `const` qualifiers in tested functions.
- `tv_list_find_str()` second argument is more in-line with other
  `tv_list_find*()` functions.

test/unit/eval/helpers.lua

@@ -405,7 +405,13 @@ local alloc_logging_helpers = {
   freed = function(p) return {func='free', args={type(p) == 'table' and p or void(p)}} end,
 }
 
+local function int(n)
+  return {[type_key]=int_type, value=n}
+end
+
 return {
+  int=int,
+
   null_string=null_string,
   null_list=null_list,
   null_dict=null_dict,