After a fix to Jenkins' wiredtiger-pull-request-linux configuration, we now have the ability to disable long tests in csuite. The wt2909_checkpoint_integrity test currently has the subtest not run if long tests are disabled. Really, the main test should be disabled if this is the case.

Secondly, if a subtest fails, we aren't particularly interested in why this is, unless there is a failure in recovery. So subtests should not create core files, as they'll just fill up the disk, and cause confusion.