Blender Git Loki

USD: Created abstract Blendfile-loading test class

This test class minimally sets up Blender so that it can load blend files
and construct a depsgraph without crashing. Note that I haven't tested this
on very complex blend files, so it may still crash when the loaded blend
file references/requires uninitialised data structures.

I also changed the USD Export Hierarchy Iterator Test to use the new test
class, and renamed it to Abstract Hierarchy Iterator Test as it doesn't
test anything USD-specific.