Fix concurrent services after install and in autopkgtests

Christian Ehrhardtrequested to merge
paelzer-guest/chrony:test-issues-focal-1870144-debian into master

This started on Ubuntu but applies to Debian just as much, therefore here an MR with a solution. TL;DR:

  • after install chrony and timesyncd are running
  • chronyd and/or timesyncd running in background makes test results flaky

References: LP-Bug: https://bugs.launchpad.net/ubuntu/+source/chrony/+bug/1870144 PPA: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/4000 Pre-Tests: https://bileto.ubuntu.com/excuses/4000/focal.html

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