The helping wrapper facilitates the usage of sharded<sstable_directory> for several test cases and the helper and its callers had deserved some cleanup over time.
Closes#12791
* github.com:scylladb/scylladb:
sstable_directory_test: Reindent and de-multiline
sstable_directory_test: Enlighten and rename sstable_from_existing_file
sstable_directory_test: Remove constant parallelizm parameter