v4 API: Improve Subscriber Tests

[n7studios]

Summary

Create a subscriber before subscribing them to a resource, such as a form, tag or sequence (vs. using an existing subscriber that is already subscribed to a resource), to improve the accuracy of tests.

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)