Better handling of email variables in test emails

[JarrydLong]

Is your feature request related to a problem? Please describe.
Certain variables in templates don't get used or replaced when sending a test email.

This specifically relates:

!!renewal_date!! in the Recurring Payment Reminder
!!membership_level_name!! is blank in the Checkout Paid template
!!membership_level_name!! is set to 'All Levels' when running a test on the Cancel template

Describe the solution you'd like
Consistent data in the email variables when testing templates.

Describe alternatives you've considered
A code recipe can work around this but will be site-specific