nixos/manual/writing-nixos-tests: document how to disable Black silently

[khumba]

Motivation for this change

This is following up on a request from @flokli in #96515 to add documentation for disabling the Black linter for NixOS tests without generating a warning (re. #72964).

I've built the manual with the two commands from the manual to verify the changes:

make -C nixos/doc/manual
nix-build nixos/release.nix -A manual.x86_64-linux

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • I don't see any, but I've run the manual's commands for itself, as above.
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Nope, Git has problems with this patch apparently and nixpkgs-review's call to it chokes. Magit was having issues too, kinda weird.
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
  • That's all this PR does :).
• Fits CONTRIBUTING.md.

[Ma27]

I actually think this is fine to use in nixpkgs if you have e.g. foo.succeed("${something-that-evaluates-to-a-store-path}") that would be short enough for one line or did I miss some recent discussion?

[flokli]

No, this is not okay, as it disables all linting for the whole file.

[flokli]

There's #72964, which proposes switching to less stricter linting, so these kind of interpolations are not too much of a problem, but we shouldn't encourage disabling linting alltogether.

[flokli]

Thanks for the PR!

[Ma27]

@flokli I thought #fmt: on re-activates it oO

[khumba]

@Ma27 # fmt: on does reactivate it past that point. Although it's the end of the file and it's not strictly needed for Black to be satisfied, I included it to work around an issue I mentioned here where if you leave it off, Black will still complain about extra newlines at the end of the file. You get those if you interpolate a '' string at the end of testScript. For the tests I've written (not in Nixpkgs), I hit that 100% of the time because I have a used a wrapper function to do that. "# fmt: off\n${testScript}" would work instead but it's not as ergonomic.

(edited to add missing quote)

[khumba]

And thanks for the suggestion @flokli!

[Ma27]

Ahh, I see. In that case it should indeed be avoided in nixpkgs, thanks for the clarification.