Enhance testing of backend

In the Matrix channel some flaws of the automatic testing were reported:

• I found it necessary to uncomment PHP_VERSION in .github/actions/run-tests/docker-compose.yml in the dut service's build args, or --create-images fails
• Also had to change PHP_VERSION from 7 to 8, or perl exploded
• I preferred not to install PHP and Composer in my host environment; used docker instead
  • After run-locally --create-images you've got what you need
  • From cookbook repo dir on host: docker run -it -w /cookbook --rm -v $PWD:/cookbook --entrypoint composer cookbook_unittesting_fpm install
• When running --create-fixture, I didn't realize initially that the version is a branch or tag in the Nextcloud server repo. It makes sense, but I didn't know, and just setting master doesn't work because Cookbook doesn't expect to run against that version.
• If I create a test fixture with sqlite, tests pass the first time only. Subsequent runs have a litany of missing table errors. Using mysql works as expected.

Start with .github/actions/run-tests/docker-compose.yml and the run-locally paths for --create-images and --create-fixture. Reproduce the reported Docker, PHP version, fixture-version, and SQLite rerun problems, then verify that image creation and repeated fixture-based test runs work without the reported errors.