Add test for fetchers

[nikromen]

Fixes #40

TODO:

• [x] tests for providers
• [x] some ez tests for schema (probably not needed)
• [x] make it work locally via makefile and create github workflow for it

[nikromen]

I'm curious, why did you create the additional level of hierarchy with the src/ subdirectory?

because I had to create tests directory. I had 2 options - do it in the repo root which doesn't make sense since there's backend and frontend dir as monorepos (and frontend also src/) and I would put tests dir for backend but outside of the backend subrepo.

The other option was to put tests directly to backend but then there will be code and then randomly a test dir :/ So I created this hierarchy - also it is easier to run these tests inside makefile since the code and tests are close to each other

Yep... it was bad idea from me at the beginning to put the code right in the backend monorepo root dir