ZTestData is Go counterpart of ztestdata on zodbtools/py side from
https://lab.nexedi.com/nexedi/zodbtools/-/blob/f9d36ba7/zodbtools/test/conftest.py#L31-65
that was recently added in nexedi/zodbtools@bf772ce0.

On Go side each ZTestData describes testdata files on the filesystem +
carry arbitrary extra information. For example FileStorage tests will
use it to place there "what is expected to load" information
corresponding to on-filesystem database.