[droidlet] Add unit-tests for refactored memory methods in memory_nodes.py

## Type of Issue

Select the type of issue:
- [ ] Bug report (to report a bug)
- [x] Feature request (to request an additional feature)
- [x] Tracker (I am just using this as a tracker)
- [ ] Refactor request
- [ ] Documentation Ask

## Description

This is an off-shoot issue based on the refactoring request in #982 and #1011. Several of the refactored methods are not being called anywhere, but also not being unit-tested. This issue adds a tracker and request for adding unit-tests for the same.

## Checklist

Use this checklist if you are using this issue as a tracker:

- [ ] https://github.com/facebookresearch/fairo/blob/fbd8f56b6bcdb776e66c2a8af90b1e6b4b87aa67/droidlet/memory/memory_nodes.py#L354
- [ ] ProgramNodes.create()
- [ ] ProgramNodes(self, memid) as a way of search
- [ ] mc_memory_nodes.VoxelObjectNodes
- [ ] _update_voxel_count
- [ ] _update_voxel_mean
- [ ] SchematicNodes
- [ ] _laod_schematics