Tests: Settings and Tagging

[n7studios]

Summary

Adds tests for the settings screen and ensuring members are subscribed and tagged.

Removes the previous wpdb->prepare() statement which was resulting in no membership levels displaying on the settings screen.

Testing

• MemberTagCest: Tests that members are subscribed and tagged in ConvertKit when configured to do so in the Plugin's settings
• SettingsCest: Tests that the settings screen works correctly when valid and invalid API keys are specified, and that membership levels can be assigned a tag.

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)