FIX: emails.rake-use domain when sending mail test (PR #12188)

rake emails:test task was always sending localhost as the domain name rather than using smtp[:domain] (aka DISCOURSE_SMTP_DOMAIN. discourse-setup has recently been modified to always set DISCOURSE_SMTP_DOMAIN, so it’s important that this test rake task actually use the value.

I tested this change on a standard production install, and it’s working as expected. Hopefully I managed not to bungle the copy/paste of the single line here in the github edit window.