Improve acceptance tests

What does this MR do?

• Reduce flakyness of acceptance tests further
• Disable xdebug in CI to increase test speed
• Retry failed tests in CI

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

Very confident

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))