def ci(request, monkeypatch):
    """Parametrize all tests for CI

    Tasks may behave differently depending on whether they are running on a
    Continuous Integration (CI) server (where the environment variable `CI`
    will be set).

    This fixture makes sure all tests exercise tasks in both scenarios.
    """
    if request.param is True:
        monkeypatch.setenv("CI", "true")
    elif request.param is False:
        monkeypatch.delenv("CI", raising=False)
    return request.param