contributing: Explain how to run common tests

As a beginning contributor, it is far from obvious where to find tests,
how to run individual tests locally for quick development, and which
tests do what. The new "Testing changes" section aims to help with all
of these.

Co-authored-by: Emily <git@emilylange.de>

CONTRIBUTING.md

@@ -61,6 +61,15 @@ Pull requests should not be squash merged in order to keep complete commit messa
 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