Factor test tools between store and fragment

Those need the driver to deal with the fact that the store is stateful. Those
tests can't be moved to the test suite because they use private functions.

libraries/persistent_store/src/lib.rs

@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Google LLC
+// Copyright 2019-2021 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -359,6 +359,8 @@ pub mod fragment;
 mod model;
 mod storage;
 mod store;
+#[cfg(test)]
+mod test;
 
 #[cfg(feature = "std")]
 pub use self::buffer::{BufferCorruptFunction, BufferOptions, BufferStorage};