ManageMail.template.php: Incorrect variable name "mailtest_result_true"

[m4z]

Description

The language files define $txt['mailtest_result_success'], so that's probably what is meant.

Steps to reproduce

1. Attempt to use the admin center feature Maintenance -> Mail -> Send Test
2. If successful, an empty message will be shown in a small green box below the menu items and above the "real" content.
3. Error log contains the following:

Type of error: Undefined 
8: Undefined index: mailtest_result_true
https://(redacted)/index.php?action=admin;area=mailqueue;sa=test;result=success
/home/(redacted)/public_html/Themes/default/ManageMail.template.php (Line 56)

Environment (complete as necessary)

• Version/Git revision: Both 2.1rc2 and current HEAD