Action hook for test emails

[ipokkel]

Is your feature request related to a problem? Please describe.
When debugging emails it is difficult to determine whether it is a test email being sent from the Memberships > Settings > Email Templates page.

Describe the solution you'd like
It would be helpful to be able to hook into the pmpro_email_templates_send_test() function, e.t. pmpro_sending_test_email, that includes the PMPro email object to assist with identifying whether it is a test email being sent and modify the data and body content for the test email.

Describe alternatives you've considered
hook into the pmpro_email_filter and strpos check for the string THIS IS A TEST EMAIL