The test-env in question is mostly started in one-shard mode. Also there
are several boost tests that start sharded<> environment. In that case
instances on different shards live in different temp dirs. That's not
critical yet, but better to have single directory for the whole test.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closesscylladb/scylladb#20412