Merge pull request #238501 from l0b0/contributing-guide-test-instructions

This means that, when addressing review comments in order to keep the pull request in an always mergeable status, you will sometimes need to rewrite your branch's history and then force-push it with `git push --force-with-lease`.

Useful git commands that can help a lot with this are `git commit --patch --amend` and `git rebase --interactive`. For more details consult the git man pages or online resources like [git-rebase.io](https://git-rebase.io/) or [The Pro Git Book](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History).

## Testing changes

To run the main types of tests locally:

- Run package-internal tests with `nix-build --attr pkgs.PACKAGE.passthru.tests`

- Run [NixOS tests](https://nixos.org/manual/nixos/unstable/#sec-nixos-tests) with `nix-build --attr nixosTest.NAME`, where `NAME` is the name of the test listed in `nixos/tests/all-tests.nix`

- Run [global package tests](https://nixos.org/manual/nixpkgs/unstable/#sec-package-tests) with `nix-build --attr tests.PACKAGE`, where `PACKAGE` is the name of the test listed in `pkgs/test/default.nix`

- See `lib/tests/NAME.nix` for instructions on running specific library tests

## Rebasing between branches (i.e. from master to staging)

From time to time, changes between branches must be rebased, for example, if the