Mock out span generation for dag.test

[o-nikolas]

Create an active span for the dag run before we call update_state on it for the first time. This way the span generation code does not run (which fails to work in the context see traceback below), we don't care about spans in the context of dag.test anyway.

Here is the exception that was thrown before this PR as a result of #43941:

devel-common/src/tests_common/test_utils/system_tests.py:68: in test_run
    dag_run = dag.test(
airflow-core/src/airflow/utils/session.py:102: in wrapper
    return func(*args, session=session, **kwargs)
airflow-core/src/airflow/models/dag.py:1705: in test
    schedulable_tis, _ = dr.update_state(session=session)
airflow-core/src/airflow/utils/session.py:99: in wrapper
    return func(*args, **kwargs)
airflow-core/src/airflow/models/dagrun.py:1197: in update_state
    self.start_dr_spans_if_needed(tis=tis)
airflow-core/src/airflow/utils/session.py:102: in wrapper
    return func(*args, session=session, **kwargs)
airflow-core/src/airflow/models/dagrun.py:968: in start_dr_spans_if_needed
    if self.span_status == SpanStatus.NOT_STARTED or self.span_status == SpanStatus.NEEDS_CONTINUANCE:
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py:487: in __get__
    return self.impl.get(state, dict_)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py:959: in get
    value = self._fire_loader_callables(state, key, passive)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py:990: in _fire_loader_callables
    return state._load_expired(state, passive)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/state.py:712: in _load_expired
    self.manager.expired_attribute_loader(self, toload, passive)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/loading.py:1369: in load_scalar_attributes
    raise orm_exc.DetachedInstanceError(
E   sqlalchemy.orm.exc.DetachedInstanceError: Instance <DagRun at 0x75a69394bbe0> is not bound to a Session; attribute refresh operation cannot proceed (Background on this error at: https://sqlalche.me/e/14/bhk3)

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[ferruzzi]

Seems like a reasonable workaround