Add a test that verifies filesystem_storage::clone preserves the sstable
state: an sstable in staging is cloned to a new generation, the clone is
re-loaded from the staging directory, and its state is asserted to still
be staging.
The change proves that https://scylladb.atlassian.net/browse/SCYLLADB-1205
is invalid, and can be closed.
* No functional change and no backport needed
Closesscylladb/scylladb#29209
* github.com:scylladb/scylladb:
test: add test_sstable_clone_preserves_staging_state
test: derive sstable state from directory in test_env::make_sstable
sstables: log debug message in filesystem_storage::clone