vendor: enable written regression tests in derivations

[Aleksanaa]

• Giving libnixf the nativeCheckInputs that should be used for nixd, which are not actually needed for libnixf. #578
  

  nixd/libnixf/default.nix

  Lines 33 to 36 in da2d7de

  	nativeCheckInputs = [
  	lit
  	nixpkgs-fmt
  	];

• Missing doCheck = true in derivations.
• Trying to pass NIX_PATH (which is non-existent in build environment) to lit, resulting in a TypeError in subprocess.Popen.  #541
• Multi-threaded tests make nix sqlite busy, causing random failures in attrs-completion.md

******************** TEST 'nixd-attrset-eval :: attrs-completion.md' FAILED ********************
Script:
--
: 'RUN: at line 1';   nixd-attrset-eval --lit-test < /build/source/nixd/tools/nixd-attrset-eval/test/attrs-completion.md | FileCheck /build/source/nixd/tools/nixd-attrset-eval/test/attrs-completion.md
--
Exit Code: 2

Command Output (stdout):
--
$ ":" "RUN: at line 1"
$ "nixd-attrset-eval" "--lit-test"
# command stderr:
warning: '/nix/var/nix' does not exist, so Nix will use '/build/.local/share/nix/root' as a chroot store
terminate called after throwing an instance of 'nix::SQLiteBusy'
  what():  �[31;1merror:�[0m SQLite database '�[35;1m/build/.local/share/nix/root/nix/var/nix/db/db.sqlite�[0m' is busy

error: command failed with exit status: -6
$ "FileCheck" "/build/source/nixd/tools/nixd-attrset-eval/test/attrs-completion.md"
# command stderr:
FileCheck error: '<stdin>' is empty.
FileCheck command line:  FileCheck /build/source/nixd/tools/nixd-attrset-eval/test/attrs-completion.md

error: command failed with exit status: 2

--

********************
********************
Failed Tests (1):
  nixd-attrset-eval :: attrs-completion.md

• 60s timeout is not actually enough for single-threaded builds, but sometimes enough (another source of random failures).
  

  nixd/nixd/tools/meson.build

  Line 34 in da2d7de

  timeout: 60,

• Regression testing nixpkgs-fmt. Instead, a script that outputs specific content should replace it, simply verifying that nixd passed the correct arguments to nixpkgs-fmt. #570

[inclyc]

The CI in this project never fails randomly.

https://github.com/nix-community/nixd/actions

60s timeout is not actually enough for single-threaded builds, but sometimes enough (another source of random failures).

Multi-threaded tests make nix sqlite busy, causing random failures in

Maybe just increase the testing time. But as you can see, it is always enough for github CI.

Regression testing nixpkgs-fmt. Instead, a script that outputs specific content should replace it, simply verifying that nixd passed the correct arguments to nixpkgs-fmt.

It won't increase the closure, right? Yes, a single script might be better.

[inclyc]

nixd/default.nix

Line 61 in da2d7de

# Disable nixd regression tests, because it uses some features provided by

[inclyc]

Reopen as it is still planned. Generally we'd like to improve testing coverage in packaging environment. Thanks @Aleksanaa