Create Tests Trait

[n7studios]

Summary

Moves method tests to a new TestsTrait which can easily be shared with the Kit WordPress Libraries, in the same way the ConvertKit_API_Traits can be easily shared with the Kit WordPress Libraries.

Other methods and test classes are specific to the PHP SDK or WordPress Libraries.

This allows for easy copy/paste of the traits and their tests from the PHP SDK to the WordPress Libraries, without editing other files.

Testing

Existing 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
• The code passes when running PHPStan
• 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)