Updates merge template: How to create a testing issue

What does this MR do?

It updates the template for MRs with a more precise definition how the test task should be posted in Slack channel.

How confident are you it won't break things if deployed?

No code changes.

Links to related issues

Hackweek discussion: https://codi.kanthaus.online/Beta-testing-workflow?view

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Take a look at changed description (this description or that one changed in the repository).

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.