sstable_directory_test: fix threading confusion in make_sstable_directory_for*() functions
sstable_directory_test: fix threading confusion in make_sstable_directory_for*() functions

The make_sstable_directory_for*() functions run in a thread, and
call functions that run in a thread, but return a future. This
more or less works but is a dangerous construct that can fail.

Fix by returning a regular value.

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