[DOCUMENTATION] Add instructions for running unit tests / CI locally

https://github.com/CosmoStat/wf-psf/blob/ab991f404e84101de4c47978f66c01723450921b/docs/source/installation.md?plain=1#L1

not sure this should go in the installation section but somewhere in the documentation we should explain how to run tests locally and silence some integration tests with the command:

`export GITHUB_ENV=False`