v4 API: Mock Create Tag Tests

[n7studios]

Summary

Mocks responses for testCreateTag and testCreateTags, with response objects matching those from the v4 API.

This prevents the ConvertKit account having hundreds of tags created across multiple tests and PRs, due to there being no API method to delete tags.

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)