Add command line option for sending test email

[mountaindude]

Discussed in #429

^{Originally posted by mountaindude May 5, 2022}
Configuring Butler's reload failure alart email feature is not difficult per se, but experience shows that it can be tricky to find the correct settings of mail server IP/host name, ports, credentials, encryption etc.

Email settings are stored in the Butler config file, but to actually use them a failed reload alerts is needed.
For example from a task that does nothing but intentionally fails.

It would be useful to have a way to send a test email from the command line, without having to go via a reload of the always-failing-app.

Something like

--send-test-email "<some.address@company.com>"