Support Flask-SQLAlchemy 3.0
    
    Since Flask-SQLAlchemy 3.0, it is necessary to manually push Flask
    application context when needed (in tests, in CLI commands not
    registered through Flask). This patch adjusts our code to set the app
    context when needed.
    
    I chose a bit different approach to Frantisek's PR #262, using pytest
    fixtures instead. While doing that, I converted all old xunit-style
    classes to pytest fixtures. One of the reasons is that xunit-style
    special methods can't receive fixture arguments, but it's not
    communicated and confuses the developer, so I converted them everywhere.
    
    Fixes: https://pagure.io/fedora-qa/blockerbugs/issue/260
    Related: https://pagure.io/fedora-qa/blockerbugs/pull-request/262