The "erased_set_get" test case deinits `FileSystemStore`, erases the
block device, and reinits `FileSystemStore`. This of course fails,
because `BlockDevice::erase()` removes all stored data including the
format of the `FileSystem` (middle layer), unless the particular type
of block device has a no-op erase implementation.
Note: Previously `HeapBlockDevice::erase()` was essentially a no-op
so this test case did not fail. We recently added the freeing of heap
memory and it uncovered the problem.