Move queue_mail_test into tests/modules

Problem/Motivation

Thanks for this module!

queue_mail_test currently lives in the root of the project source tree.
Core's convention is to put test modules in tests/modules/*.
That keeps other directories clean and easier to understand.
Sites that do hardening that removes tests directories from all projects in their production filesystem will always clean them up.

Proposed resolution

% git mv queue_mail_test/ tests/modules

Remaining tasks

1. Agree to do it.
2. Do it:
   

   mkdir tests/modules
   git mv queue_mail_test/ tests/modules
   # use git commit command from the issue
   git push
   

Thanks!
-Derek