Tests for the volatile DB

The set of commands for the volatile DB is similar to the immutable DB, commands such as:

• Get a block or information about a block
• Add a block
• Simulate disk corruption

in addition to a few commands that are supported only by the volatile DB, such as "find all blocks with the given predecessor" (used by chain selection). The model (defined in Test.Ouroboros.Storage.VolatileDB.Model) is a list of "files", where every file is modelled simply as a list of blocks and some block metadata. The reason that this is slightly more detailed than one might hope (just a set of blocks) is that we need the additional detail to be able to predict the effects of disk corruption.