Include LocalStorageService in the testing documentation.

[aozarov]

Currently LocalStorageService is being implemented in the gcs-nio branch.
Once that branch is merged and LocalStorageService is ready for use we should make sure
it has its own package-info.java and update the TESTING instructions to include it.

/cc @jart @jean-philippe-martin

[garrettjonesgoogle]

It doesn't appear that this ever reached master - closing out the issue.

[jean-philippe-martin]

LocalStorage was renamed to LocalStorageHelper but it's still very much around and super useful for tests. It is still the case that it would be nice to expose it to a broader audience once we feel it's ready, as we've had informal inquiries from others who also would like a fake storage option to test their own storage-using code.

[garrettjonesgoogle]

Ahh ok, thanks for letting me know its new location :-)

[lesv]

We have all the others, we should have this as well.