Unify shared examples for chat notification integrations.

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

There are two shared examples for chat notification integrations

• Integrations::SlackMattermostNotifier
• "chat integration"

Both have in common that the #execute method needs to be tested for all kinds of events, so it might be possible to DRY up those examples.