Test/format already has leader mode for disagg WT-14530.  That ticket hardwires disaggregated(role=leader,....) into the configuration string.  For ticket, we'll want to have a test/format configuration (see test/format/config.sh) that sets the initial role for disagg to leader or follower.  (In the future they'll be at least one additional mode added, probably switch).  When the mode is follower, we want the WT configuration string to have  disaggregated(role=follower,....).  

Then of course we want to run it.  test/format will probably run a short time in follower mode, as that mode is entirely in memory, so workloads will be limited by cache size.  But we could do a small number of inserts and mostly modifications to make longer workloads.

Ideally this work would include adding some evergreen testing of follower mode.