In order to test tiered storage with the range of other WiredTiger configuration options, workload permutations, etc., we need to add support for it to test/format.

Initially we will want to include:

• A configuration option specifying that the test should use tiered table(s)
• Configuration option(s) using different ways of triggering flush operations (periodically from the application, periodically managed by WT, wait vs. nowait, timeout, etc.)
• Tiered manager configuration options

We can expand this list as we think of other things.

Note that flush_tier is an option on checkpoint.  Other test programs (test/timestamp_abort, test/schema_abort) have been refactored so the checkpoint thread uses testutil_tiered_sleep instead of sleep to help determine when to incorporate the flush option.  That may be helpful here as well.