Move Namespaces to use Statements

[n7studios]

Summary

Tidies up test code by adding use statements for ClientException, Dotenv and the ConvertKit_API classes.

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)