Add 2.3.1 to the update tests and update the downgrade target for the
downgrade version. This commit also fixes two issues that were fixed in
the release branch:
1. Drop `_timescaledb_internal.refresh_continuous_aggregate`
2. Update the changelog to match the release branch.
In order to implement repair tests, changes are made to the
`constraint_check` table to simulate a broken dependency, which
requires the constraints on that table to be dropped. This means that
the repair runs without constraints, and a bug in the update test could
potentially not get caught.
This commit fixes this by factoring out the repair tests from the
update tests and run them as a separate pass. This means that the
contraints are not dropped in the update tests and bugs there will be
caught.
In addition, some bash functions are factored out into a separate file
to avoid duplication.
