Tests for the DB marker and DB lock.

When the consensus layer is integrated into the main node, it provides two safe guards to avoid data loss and/or corruption:

• When the database is opened, it locks the database so that there can be no other processes trying to access it at the same time.
• When we create a database directory, we place a "magic marker" in that directory. This allows us to distinguish the database directory from other directories, and avoids that we would try to "truncate" a "chain" in a directory which doesn't contain a DB at all (due to a misconfiguration), thereby potentially deleting a user's files.

This module contains a bunch of unit tests to make sure that these locks and markers are created correctly and behave as expected.