[CI] Add blob unit tests

Summary:
Add blob C++ unit tests to CI

Currently there are no C++ tests which don't rely on aws, so just make the
ability to do unit tests in the future a possibility.

https://linear.app/comm/issue/ENG-1789

Test Plan: https://buildkite.com/comm/blob-nix/builds/57#01835777-7eeb-469e-8138-d5ba04e88d4e passes

Reviewers: atul, varun, abosh

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5181