`TestForumInternals` assumed that only the base forum and the forum created by the test setup existed in the database.

This is not true when tests are run with demo data. In particular, `website_slides_forum` creates additional demo forums, causing `test_assert_initial_values` and the forum count assertions to fail even though the forum behavior itself is correct.

This commit avoids relying on the global number of existing forums and instead scope the initial-value assertions to the forums owned by the test itself.

For forum count tests, we use the count observed at the beginning of the test as a baseline and assert the expected deltas after creating or updating forums. This keeps the test focused on the behavior being tested while allowing unrelated forums to already exist.

This also avoids modifying, archiving, or explicitly filtering demo records, and does not introduce a dependency on modules such as `website_slides_forum`.

The tests therefore behave identically on databases both with and without demo data.

[error-241100
](https://runbot.odoo.com/odoo/error/241100)


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