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.

src/nvim/ex_docmd.c

@@ -8424,7 +8424,7 @@ eval_vars (
           return NULL;
         }
         result = (char_u *)tv_list_find_str(get_vim_var_list(VV_OLDFILES),
-                                            (long)i);
+                                            i - 1);
         if (result == NULL) {
           *errormsg = (char_u *)"";
           return NULL;