Store snapshots from static inner test classes into own sub directory of parent class

[skuzzle]

Currently, we just use the full class name as directory name. That leads to directories named like ParentClass$StaticInnterClass_snapshots. We should deconstruct the class name and create proper sub directories for the inner classes.

When doing this, we should also provide an automatic migration of existing snapshot files. Otherwise we might break existing tests and a users might need to inconveniently migrate existing snapshot files manually.