add feature-flags on test files

this allows to run 'cargo test' with only a subset of features enabled,
i.e.
  cargo test
or
  cargo test --features sled

lib/tests/rocksdb_store.rs

@@ -1,3 +1,5 @@
+#![cfg(features = "rocksdb")]
+
 use oxigraph::model::vocab::{rdf, xsd};
 use oxigraph::model::*;
 use oxigraph::RocksDbStore;

lib/tests/sled_store.rs

@@ -1,3 +1,5 @@
+#![cfg(features = "sled")]
+
 use oxigraph::io::{DatasetFormat, GraphFormat};
 use oxigraph::model::vocab::{rdf, xsd};
 use oxigraph::model::*;