Add support to verify tiered storage contents to tiered_abort

While working on WT-9085, we discussed having more verification in tiered_abort. Right now, similar to all the other *_abort programs it checks for the correct contents. However for this test it would be good to add a directory content verification.

We need to check what exists where after the child process is killed, and then recheck after the flush_tier that the parent runs to restart any interrupted flushes. Possibly this work should also sleep for the local_retention time (or set it very small) so that all newly flushed objects are removed from the local directory.