pkgs-lib/tests/formats: improve message on failure

Foremost, the message was discarding double quotes on one side of the
diff, which was super-confusing to me, as I thought that the format
convertor broke that when in fact only whitespace was changed.

I thought I'd cat the files, but then... switching to `diff -u` seemed
self-sufficient.  It felt sufficiently non-controversial to push
directly, but certainly feel free to improve further.

pkgs/pkgs-lib/tests/formats.nix

@@ -15,11 +15,11 @@ let
     in formatSet.generate "test-format-file" config;
 
   runBuildTest = name: { drv, expected }: pkgs.runCommandNoCC name {} ''
-    if diff ${drv} ${builtins.toFile "expected" expected}; then
-      touch $out
+    if diff -u '${builtins.toFile "expected" expected}' '${drv}'; then
+      touch "$out"
     else
-      echo "Got: $(cat ${drv})"
-      echo "Should be: ${expected}"
+      echo
+      echo "Got different values than expected; diff above."
       exit 1
     fi
   '';