Tests: Retry apiCheckSubscriberExists

[n7studios]

Summary

Adds a retry method when asserting if a subscriber exists, as sometimes the API takes a few seconds to reflect that a new subscriber has been added.

Moves wait calls into the API helper methods.

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
• 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)