Tests: Fix testCreateFormWithInvalidCustomField

[n7studios]

Summary

Reverts #144, now that creating a subscriber with invalid custom fields via the API will create the subscriber, rather than returning a non-2xx HTTP response.

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
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• Security and Sanitization requirements have been followed
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