nix-unit: init at 2.18.0

This is now used by several projects including dream2nix.
Having not to compile from source is a big win.

pkgs/by-name/ni/nix-unit/package.nix

@@ -0,0 +1,57 @@
+{ stdenv
+, lib
+, boost
+, clang-tools
+, cmake
+, difftastic
+, makeWrapper
+, meson
+, ninja
+, nixVersions
+, nlohmann_json
+, pkg-config
+, fetchFromGitHub
+}:
+
+stdenv.mkDerivation (finalAttrs: {
+  pname = "nix-unit";
+  version = "2.18.0";
+
+  src = fetchFromGitHub {
+    owner = "nix-community";
+    repo = "nix-unit";
+    rev = "v${finalAttrs.version}";
+    hash = "sha256-9wq14p+85oW4HlD42NJ0jyA++z3nEYjFQ6uT40xdfbc=";
+  };
+
+  buildInputs = [
+    nlohmann_json
+    # We pin the nix version to a known working one here as upgrades can likely break the build.
+    # Since the nix language is rather stable we don't always need to have the latest and greatest for unit tests
+    # On each update of nix unit we should re-evaluate what version we need.
+    nixVersions.nix_2_18
+    boost
+  ];
+
+  nativeBuildInputs = [
+    makeWrapper
+    meson
+    pkg-config
+    ninja
+    # nlohmann_json can be only discovered via cmake files
+    cmake
+  ] ++ lib.optional stdenv.cc.isClang [ clang-tools ];
+
+  postInstall = ''
+    wrapProgram "$out/bin/nix-unit" --prefix PATH : ${difftastic}/bin
+  '';
+
+  meta = {
+    description = "Nix unit test runner";
+    homepage = "https://github.com/nix-community/nix-unit";
+    license = lib.licenses.gpl3;
+    maintainers = with lib.maintainers; [ mic92 adisbladis ];
+    platforms = lib.platforms.unix;
+    mainProgram = "nix-unit";
+  };
+})