tests: Many functional tests are skipped now

When I run all functional tests (in testing container with latest package updates), most of the apps are being skipped, for example:

plinth/modules/sogo/tests/test_functional.py::TestSOGoApp::test_enable_disable SKIPPED [ 62%]
plinth/modules/sogo/tests/test_functional.py::TestSOGoApp::test_run_diagnostics SKIPPED [ 62%]
plinth/modules/sogo/tests/test_functional.py::TestSOGoApp::test_backup_restore SKIPPED [ 62%]
plinth/modules/sogo/tests/test_functional.py::TestSOGoApp::test_uninstall SKIPPED [ 62%]
plinth/modules/sogo/tests/test_functional.py::TestSOGoApp::test_login SKIPPED [ 62%]
...
  /freedombox/plinth/tests/functional/__init__.py:471: UserWarning: App sogo is not available in distribution

If I manually go to the SoGO page, then I can install it without any issue. I think the tests are confused by the recent changes to checking app availability.