v4 API: Tests: Run wordpress endpoint tests

[n7studios]

Summary

Runs tests using the wordpress endpoint, that were previously marked as incomplete, resolving:

• Reinstating the missing recommendations_script method, that previously existed.
• Adding an api_key parameter to get_form_html for fetching legacy forms, as the v4 API does not use API Keys or list legacy forms. This method may be used by the main ConvertKit Plugin to support existing legacy forms that were previously defined in the Plugin or a shortcode/block.
• Removes testBackwardCompat* tests for methods that were marked deprecated in v1 of our WordPress Libraries, as these functions are removed in this v2 WordPress Library.

Testing

Tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)