add missing file and callpackage as import

packages/hello/default.nix

@@ -13,6 +13,7 @@
     stdenv,
     fetchurl,
     nixos,
+    callPackage,
     testers,
     versionCheckHook,
     hello,

packages/hello/test.nix

@@ -0,0 +1,8 @@
+{ runCommand, hello }:
+
+runCommand "hello-test-run" {
+  nativeBuildInputs = [ hello ];
+} ''
+  diff -U3 --color=auto <(hello) <(echo 'Hello, world!')
+  touch $out
+''