Add CI tests for python backend

[mylibrar]

Is your feature request related to a problem? Please describe.
Currently Stave repo only has a single backend test and it is not integrated into the GitHub workflow.

Describe the solution you'd like
We want a comprehensive backend test that consists of unit tests validating all the major functionalities of Stave backend. These tests should be included into CI workflow such that every PR will be checked upon each new commit.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.