Avoid modifying global category/filters in tests

[devos50]

In the metadata store tests, we are modifying default_xxx_filter and add a term to it. This change is persistent across different tests, potentially leading to test failures. This PR modifies the tests and increases flexibility by creating separate instances of Category and XXXFilter. These objects are manipulated without affecting default_category_filter or default_xxx_filter.

Fixes #6500 (hopefully)

[kozlovsky]

Looks good! It is better to have tests isolated by avoiding of global variables.

I think the actual reason for the error in #6500 was this line:

async def test_get_my_channel_tags_xxx(metadata_store, tags_db, mock_dlmgr_get_download, my_channel, rest_api):  # pylint: disable=redefined-outer-name
    """
    Test whether XXX tags are correctly filtered
    """
    with db_session:
        ...
        default_xxx_filter.xxx_terms = {"wrongterm"}

If it was executed before tests in test_category.py it could destroy the expected default_xxx_filter content.

I think it should be fixed as well (in this or separate PR)

[sonarqubecloud]

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[devos50]

@kozlovsky thanks for pointing that out! I tried to see if there is an easy way to get rid of the default_category_filter and default_xxx_filter variables and inject them in the components. This way, we should also be able to replace entries in the filter during the tests. This, however, turned out to be much work than I anticipated since the category filter is integrated in the metadata store and not easy to replace on runtime.

We could consider making a component out of the category filter and pass it to the components that need it (e.g., REST endpoints/metadata store). I'm not sure how much work that is but this is more something for a subsequent PR.