Mock CellEndpoint rather than CellAdapter in HsmRestoreTest

Now that most of our code no longer depends directly on CellNucleus and
CellAdapter, we only have to mock CellEndpoint.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6760/