Test Notifications

I've standardised on Notifications because it's Simplenews++, but one of the features Simplenews had that Notifications doesn't (as far as I can find) is the ability to send test messages. My clients find this feature _very_ reassuring, especially if they're about to send a message to hundreds of people.

Here's a rough first pass at a module to implement this. Limitations are:

• It only works with Content Notifications.
• It uses Notifications Lite (because I found the full Notifications API so bewilderingly complex) so if you're using Text Templates, you'll have to adjust the "Simple Notifications" templates to match the templates used for the events you're testing to get a true preview of the "real" notifications.
• It assumes you'll usually want to use the same test recipients every time, so it stores these as a system variable rather than per-node. That suits my use cases, but maybe not everybody's.
• Many other limitations probably exist.