[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