Tests: Update Tests to match API behaviour

[n7studios]

Summary

Updates existing tests:

• Tags: creating a tag that exists returns the tag (previously, the API would return an error)
• Before / after params: set sensible before/after dates, now the test Kit account has had the majority of its test subscribers removed
• Apply the timezone offset (-04:00 or -05:00) depending on whether daylight savings time is active

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)

[noelherrick]

@n7studios Have you communicated to the Platform team that there's an error in the API already?

[n7studios]

@n7studios Have you communicated to the Platform team that there's an error in the API already?

Yes - waiting to hear back. Will hold off to confirm whether the 500 error on bulk/tags with blank tag names is a bug or expected behaviour.

[n7studios]

500 error from the API resolved in this PR; reverted test accordingly.