Remove (at least temporarily) the test for warnings from isolation level

There are two problems with this test:

* `SET TRANSACTION` ... does not make sense except after `BEGIN`.
* The exact wording of the warning has changed.

Note: In psql to real Postgres, if you do `SET TRANSACTION ISOLATION MODE x`, it succeeds because it executes that single statement in its own transaction. `SHOW` confirms that it didn't do anything. I don't think this is helpful, but we could do that if it seems important to compatibly ignore this.

I'm agreeable with fixing it and putting it back after things settle down, though it does not seem to me to add any unique testing to the CLI's handling of warnings.