Add tests for EventForm

In addition to adding tests of the fields for
`xmas.events.forms.EventForm`, a test settings module is being added.
This module can be imported and passed to `xmas.factory.create_app()` to
override the regular application settings.
Default SSL and TLS for Flask-Mail to True

It's always better to explicitly turn off the secure options than it is
to default to using the insecure ones.