Add a trivial "load test"

Problem/Motivation

We could use some phpunit tests for this module.

A very quick/simple initial win would be a Functional test that installs group_notify, tries to load the front page, and makes sure we don't blow up. ;)

This will catch things like:
- PHP fatal errors.
- Config schema mis-matches.
...

Proposed resolution

Add a trivial tests/src/Functional/LoadTest.php test for group_notify.

Remaining tasks

1. Upload.
2. Commit.