Notify of test failure via slack

[IgorTatarnikov]

We've added a step to most BrainGlobe CI workflows to notify us via slack if scheduled tests fail. This makes sure that scheduled test failures are made known to anyone following a specific slack channel.

Should this be added to the test action, or should we maintain a wrapper around https://github.com/ravsamhq/notify-slack-action to allow us to change settings without having to update every single repository?