Previous to pulling out and extending the fake backend, it was quite
cumbersome to write tests for very specific scenarios regarding
backends. But as we have made it more generic, it has become much easier
to do so. As such, this commit adds multiple tests for scenarios with
multiple backends for the ODB.
The changes also include a test for a very targeted scenario. When one
backend found a matching object via `read_prefix`, but the last backend
returns `GIT_ENOTFOUND` and when object hash verification is turned off,
we fail to reset the error code to `GIT_OK`. This causes us to segfault
later on, when doing a double-free on the returned object.
