nixos/activation-script: Add lib.sh with warn()

nixos/modules/system/activation/activation-script.nix

···
     ''
       #!${pkgs.runtimeShell}
 
+      source ${./lib/lib.sh}
+
       systemConfig='@out@'
 
       export PATH=/empty

nixos/modules/system/activation/lib/lib.sh

···
+# shellcheck shell=bash
+
+warn() {
+    printf "\033[1;35mwarning:\033[0m %s\n" "$*" >&2
+}

nixos/modules/system/activation/lib/test.nix

···
+# Run:
+#   nix-build -A nixosTests.activation-lib
+{ lib, stdenv, testers }:
+let
+  inherit (lib) fileset;
+
+  runTests = stdenv.mkDerivation {
+    name = "tests-activation-lib";
+    src = fileset.toSource {
+      root = ./.;
+      fileset = fileset.unions [
+        ./lib.sh
+        ./test.sh
+      ];
+    };
+    buildPhase = ":";
+    doCheck = true;
+    postUnpack = ''
+      patchShebangs --build .
+    '';
+    checkPhase = ''
+      ./test.sh
+    '';
+    installPhase = ''
+      touch $out
+    '';
+  };
+
+  runShellcheck = testers.shellcheck {
+    src = runTests.src;
+  };
+
+in
+lib.recurseIntoAttrs {
+  inherit runTests runShellcheck;
+}

nixos/modules/system/activation/lib/test.sh

···
+#!/usr/bin/env bash
+
+# Run:
+#   ./test.sh
+# or:
+#   nix-build -A nixosTests.activation-lib
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+set -euo pipefail
+
+# report failure
+onerr() {
+  set +e
+  # find failed statement
+  echo "call trace:"
+  local i=0
+  while t="$(caller $i)"; do
+    line="${t%% *}"
+    file="${t##* }"
+    echo "  $file:$line" >&2
+    ((i++))
+  done
+  # red
+  printf "\033[1;31mtest failed\033[0m\n" >&2
+  exit 1
+}
+trap onerr ERR
+
+source ./lib.sh
+
+(warn hi, this works >/dev/null) 2>&1 | grep -E $'.*warning:.* hi, this works' >/dev/null
+
+# green
+printf "\033[1;32mok\033[0m\n"

nixos/tests/all-tests.nix

···
   esphome = handleTest ./esphome.nix {};
   etc = pkgs.callPackage ../modules/system/etc/test.nix { inherit evalMinimalConfig; };
   activation = pkgs.callPackage ../modules/system/activation/test.nix { };
+  activation-lib = pkgs.callPackage ../modules/system/activation/lib/test.nix { };
   activation-var = runTest ./activation/var.nix;
   activation-nix-channel = runTest ./activation/nix-channel.nix;
   activation-etc-overlay-mutable = runTest ./activation/etc-overlay-mutable.nix;