Add ubipop integration tests

[JayZ12138]

Problem Description:
Currently, almost all tests are unit tests, though they worked well and provided good coverage, there could still be corners be omitted and leading to failure during QE. We should provide better integration tests to cover the whole population process.

Solution
The integration tests should be similar to what we have done to test pub workers:

1. should be able to test the whole ubi population process with different input sets.
2. tests shouldn't rely on external resources, such as a real server, requests, pa_tool. Instead, should mock them to exclude the influence from them.

Acceptance criteria:

1. Setup the mocked test environment
2. Add test cases to run the complete ubi population process with different input sets.

[alexandrevicenzi]

@JayZ12138 there are integration tests here.

These tests test the whole ubipop process, the only missing thing is the mocked test env.