odb: failing test for a tracing backend

If we create a fake writestream, we choose a fixed backend to write to
and don't fall back on PASSTHROUGH.
odb: try all backends when using a fake wstream

We use a fake wstream because no backends were able to support it. When
finalizing the write, we should not pin a particular backend but try
again to write, as though we were doing git_odb_write().