Tests: Fix Subscriber Checks

[n7studios]

Summary

Applies the PR from the main Kit Plugin:

Tests failed intermittently on GitHub Actions when checking a subscriber was created. Kit's API is eventually consistent - searching by email after a write can take 30 seconds to 5 minutes to catch up, however tests search immediately, resulting in failures.

Fixes tests that only know the subscriber email by adding a test-only WordPress mu-plugin that:

• Records the Kit API requests and responses
• Extracts the ID from the log
• Fetches the subscriber by ID vs. by email

In the future, these test helper methods ideally will be in the Kit WordPress Libraries, given we use them across all Kit Plugins.

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)