Test email variables 3166

[derekashauer]

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Fixes issues from #3166
Fixes issues with test template email sending returning as "failed" in admin
Adding missing membership level data in checkout free/paid email templates
Adding missing template variable !!instructions!!

How to test the changes in this Pull Request:

1. Go to any of the Email templates and send a test

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

• Fixed test emails where data was empty as per Better handling of email variables in test emails #3166
• All test emails were returning as "failed" in admin because they did not return the send() method result as the ajax response
• Including missing membership level data in checkout free/paid email templates
• Added missing template variable for !!instructions!! for check payments email