This test checks that we are actually walking over the pieces when
starting the cache, and that it is returning expected values.
A recent outage was partially caused by the fact that this cache was
accidentally reading itself (via the pieces store, which has the cache
embedded). This test ensures that does not happen, and checks that when
the cache's `Run` method is called, the space used values are read from
disk and accurately update the cache.
Change-Id: I9ec61c4299ed06c90f79b17de3ffdbbb06bc502e