Like all test/checkpoint, timestamp_abort, schema_abort, we need to have a "tiered storage" mode for test/format. When enabled, this adds extra configuration for a wiredtiger_open call: we'll need to specify the storage_source (e.g. dir_store or s3_store) and load the appropriate extension.  We'll also need to indicate how often flush_tier happens.  Probably easiest to set to frequency to in terms of "once every N checkpoints".