Add page to test mail settings

When configuring the email settings it may me handy to test them in an
easy way.
Let's add the page /dashboard/system/mail/method/test_settings
Rename from "Test SMTP" to "Test Mail"...

...since we're testing the mail system, not the SMTP settings ;)
Merge branch 'add-mail-test' of https://github.com/mlocati/concrete5 into mlocati-add-mail-test
Merge branch 'master' of github.com:concrete5/concrete5