feat(testing): improve `create_req` and `create_asgi_req`

[iuliachiriac]

Summary of Changes

This PR changes create_req and create_asgi_req to make their behaviour consistent with simulate_request by:

• adding missing parameters: content_type, json, params, params_csv
• adding call to _prepare_sim_args function (which was also moved to helpers.py), which does a series of checks on request attributes and sets Content-Type
• enabling passing arbitrary WSGI/ASGI envs through extra argument

Related Issues

https://github.com/falconry/falcon/issues/1793

Pull Request Checklist

• [x] Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• [x] Added tests for changed code.
• [ ] Prefixed code comments with GitHub nick and an appropriate prefix.
• [x] Coding style is consistent with the rest of the framework.
• [ ] Updated documentation for changed code.
  • [x] Added docstrings for any new classes, functions, or modules.
  • [x] Updated docstrings for any modifications to existing code.
  • [x] Updated both WSGI and ASGI docs (where applicable).
  • [x] Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • [x] Updated all relevant supporting documentation files under docs/.
  • [ ] A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • [ ] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• [ ] Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Run towncrier --draft to ensure it renders correctly.)

If you have any questions to any of the points above, just submit and ask! This checklist is here to help you, not to deter you from contributing!

PR template inspired by the attrs project.

[iuliachiriac]

I submitted this as a draft because I plan on addressing remaining changes in https://github.com/falconry/falcon/issues/1793 in the following days.

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (c124e3a) to head (0b23def). Report is 42 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2249 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 63 63 Lines 7112 7124 +12 Branches 1260 1264 +4 ========================================= + Hits 7112 7124 +12 ```

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[vytas7]

Hi Iuliana, and thanks once again for Sprinting in Prague! Just checking if you are planning to continue on this issue? It looks like it is not that much left here, well done so far! :100:

[iuliachiriac]

Hi Iuliana, and thanks once again for Sprinting in Prague! Just checking if you are planning to continue on this issue? It looks like it is not that much left here, well done so far! 💯

Hi Vytautas! Thank you for the opportunity and the guidance! I will get back to it this week, if it't not too late, sorry for the delay!