tests: perform code formatting test in test suite

This should improve visibility when CI job fails due to bad
formatting.

.gitlab-ci.yml

@@ -11,7 +11,6 @@ stages:
 Run tests:
   stage: test
   script:
-    - ./format -c && echo Format is OK
     - nix-shell --pure tests -A run.files-text
   rules:
     - if: $CI_COMMIT_BRANCH == "master"

.travis.yml

@@ -5,6 +5,5 @@ os:
   - osx
 
 script:
-  - ./format -c
   - nix-shell . -A install
   - nix-shell --pure --max-jobs 10 tests -A run.all

tests/default.nix

@@ -59,6 +59,7 @@ import nmt {
     ./modules/programs/zsh
     ./modules/xresources
   ] ++ lib.optionals pkgs.stdenv.hostPlatform.isLinux [
+    ./meta # Suffices to run on one platform.
     ./modules/misc/debug
     ./modules/misc/pam
     ./modules/misc/xdg

tests/meta/default.nix

@@ -0,0 +1 @@
+{ meta-formatting = ./formatting.nix; }

tests/meta/formatting.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  pinnedNixpkgs = builtins.fetchTarball {
+    url =
+      "https://github.com/NixOS/nixpkgs/archive/05f0934825c2a0750d4888c4735f9420c906b388.tar.gz";
+    sha256 = "1g8c2w0661qn89ajp44znmwfmghbbiygvdzq0rzlvlpdiz28v6gy";
+  };
+
+  pinnedPkgs = import pinnedNixpkgs { };
+
+in {
+  config = {
+    nmt.script = ''
+      PATH="${with pinnedPkgs; lib.makeBinPath [ findutils nixfmt ]}:$PATH"
+      cd ${../..}
+      if ! ${pkgs.runtimeShell} format -c; then
+        fail "${''
+        Expected source code to be formatted with nixfmt but it was not.
+        This error can be resolved by running the './format' in the project root directory.''}"
+      fi
+    '';
+  };
+}