Refactor url parser test code

[rmisev]

• Introduce string_or_null, parsed_obj and parsed_obj_with_failure classes and simplify run_parser_tests, run_host_parser_tests and run_idna_v2_tests functions.
• Fix test code to catch and report JSON file errors

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.66%. Comparing base (7734cc8) to head (08a49a5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #58 +/- ## ======================================= Coverage 99.66% 99.66% ======================================= Files 16 16 Lines 2369 2369 Branches 384 385 +1 ======================================= Hits 2361 2361 Misses 8 8 ```

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