[22.01] Fix model discovery unit tests

[nsoranzo]

that I broke when merging #13552 forward.

Fix:

app = <galaxy.model.unittest_utils.data_app.GalaxyDataTestApp object at 0x7f0e00dd3640>, target = '/tmp/tmpyj5d8268', work_directory = '/tmp/tmp_a7e191v'

    def _import_directory_to_history(app, target, work_directory):
        sa_session = app.model.context

        u = model.User(email="collection@example.com", password="password")
        import_history = model.History(name="Test History for Import", user=u)

        sa_session = app.model.context
        sa_session.add_all([u, import_history])
        sa_session.flush()

        assert len(import_history.datasets) == 0

        import_options = store.ImportOptions(allow_dataset_object_edit=True)
>       import_model_store = store.get_import_model_store_for_directory(target, app=app, user=u, import_options=import_options, tag_handler=app.tag_handler.create_tag_handler_session())
E       AttributeError: 'GalaxyDataTestApp' object has no attribute 'tag_handler'

test/unit/data/model/test_model_discovery.py:235: AttributeError

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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