The bone collection unit tests in Python were failing, but this failure
was not propagated to `ctest` and thus went unnoticed.
Both issues are now fixed.
Instead of moving bone collections by absolute index, they can now be
moved by manipulating `.child_number`. This is the relative index of the
bone collection within the list of its siblings.
This replaces the much more cumbersome `collections.move_to_parent()`
function. Since that function is now no longer necessary (it's been
replaced by assignment to `.parent` and `.child_number`), it's removed
from RNA. Note that this function was never part of even a beta build of
Blender.
Moving a bone collection to another parent is now possible in Python by
assigning to `bone_collection.parent`.
Thanks to Sergey for the implementation.
Add a `bone_collection.bones_recursive` property that returns the set of
bones assigned to that bone collection or any of its child collections.
This property is implemented in Python, as that made it considerably
simpler to create the set semantics.
Use `bpy.ops.wm.read_homefile(use_factory_startup=True)` to load the
default 'homefile'. Otherwise Blender will load the saved-as-default file,
which can be quite different from the default startup file.
Add a read-only property `bone_collection.parent` to RNA that returns
the parent bone collection.
This performs two scans of the array (one to find the bone collection's
index, and the other to find the parent index). This might look bad, but
as long as `Object.children` still loops, in Python, over all of
`bpy.data.objects`, this should also be acceptable.
Add a Python unit test that covers some of the armature/bone collections.
Some of these tests cover the same functionality as the C++ tests, albeit
via the Python/RNA API.
Another test was added to check that joining armatures works as expected.
