Improve DB migration tests in CI

### Body Our DB migration tests in CI run on empty tables and some of the migrations perform some data manipulation during down/up migration. By having empty tables we are skipping over this logic. We should likely add some steps before and in-between the migration to populate the database witth data - likely by running a few dags maybe runniing a few other commands that modify the data (connections/variables etc.). Also, it seems that some of the migrations might behave differently depending on what is the "to" migration - for example in case of 2.9.0 -> 2.8.4 migration, the original migration in #38743 failed, while 2.0.0 did not and we have to either change our CI testing or (I'd say more likely) make it a bit more "Target independent" ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.