Add unit test for the new distributed partitioning pipeline

[jimjlu99]

🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

Description

Add unit test for:

• script that converts MAG240 to the chunked graph data format
• script that conducts random partitioning on chunked graph data format
• script that conducts data dispatching

Depending work items or issues

• [x] #4276
• [x] #4192
• [ ] #4346