We currently don't have any tests that directly exercise the tree
parser. This is due to the fact that the parsers for raw object data has
only been recently introduce with commit ca4db5f4 (object: implement
function to parse raw data, 2017-10-13), and previous to that the setup
simply was too cumbersome as it always required going through the ODB.

Now that we have the infrastructure, add a suite of tests that directly
exercise the tree parser and various edge cases.