Once we write an object to a storage source, no other system (even our own) should be able to overwrite the object.  WT does a reasonable job of protecting against that, as when we create a tiered table, we check that there are no conflicting names in the cloud.  However, one can come up with situations for which that wouldn't be enough.  For example, system A creates tiered table X, which system B creates tiered table X.  Neither has done a flush_tier, so both creates "pass".  Then they each do a flush_tier in turn.  Currently dir_store does not complain about the overwrite.  Attaching a test case that uses dir_store to emulates multiple systems sharing cloud storage by "sym-linking" the bucket directories.

This ticket will develop a test case for dir_store.  Followup tickets will extend the test for real cloud storage.