pkg/blobserver/storagetest: Implement tests for blobserver.Salter.
pkg/blobserver/storagetest: Implement tests for blobserver.Salter.

This patch adds a test for the blobserver.Salter interface to the
common storage test. Any blobserver that has implemented this
interface and calls storagetest.Test() will have its salter
implementation automatically tested.

Currently, localdisk and diskpacked run this new test.

This CL depends on CL #1662: https://camlistore-review.googlesource.com/#/c/1662

Change-Id: I6e8d9bc958d3fd59bac88194e08e1b9a6ae13bc7

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