[EPIC-55] Setup Unit test for codebase

[nvatuan]

• Epic: [EPIC] Improve Infra (CI/CD, linter, unittest) for bkdnOJ.v2 bkdnOJ-v2#55

Description

• Setup unit test structure
• Setup unit test script and CI/CD, coverage
• Note: this task does not implement full unittest. We will slowly bring up coverage threshold as time goes.