Make the disk cache unit tests use some common code from the disk cache.
Make the disk cache unit tests use some common code from the disk cache.

In particular, this removes the last piece of code on the disk cache that
uses SHFileOperation.


git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@1135 0039d316-1c4b-4281-b951-d872f2087c98

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