Right now, the fake backend is quite restrained in the way how it
works: we pass it an OID which it is to return later as well as an error
code we want it to return. While this is sufficient for existing tests,
we can make the fake backend a little bit more generic in order to allow
us testing for additional scenarios.

To do so, we change the backend to not accept an error code and OID
which it is to return for queries, but instead a simple array of OIDs
with their respective blob contents. On each query, the fake backend
simply iterates through this array and returns the first matching
object.