In WT-6075, we disabled lower isolation levels in test_wt2323_join_visibility as they don't work at the time. Enable them again when they are fixed.

Before submitting a PR, run the test in a loop for several hours to ensure there is no fallout from the change. If there is fallout:

• create new Jira tickets or update existing ones as necessary.
• mark this ticket as dependent on the above failures.
• return this ticket to the backlog.