This is a follow up to WT-9423 specifically for backup and tiered storage.

We have decided that, for now, trying to open a backup cursor on a connection that is using tiered storage will return ENOTSUP. Right now it tries to do something "reasonable" with the items that exist locally, but doesn't really succeed.

This is a heavier hammer than required as it will also dis-allow backup of log files only or target= backups of individual (local) tables. But no one is using tiered storage and we can address exceptions to the heavy hammer when necessary.