Add the ability to only read a sub-tree of a tree via `Object.read_at`.
Expose the functionality via the `Store{Server,Client}.read_tree_at`.
Extend the tests to check this new functionality.
Instead of using string interpolation and concatenation to build
file system paths, use `os.path.join` or directly the constructor
for `pathlib.Path`, which can take path segments.
Move `test_objectstore` into the module-level tests. This allows us to
run it as part of `make test-module.
Make sure to properly guard it as root-only module.
