tests: sstable_test: add test_using_working_sst helper
tests: sstable_test: add test_using_working_sst helper

In preparation for sstables::test_env

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla