fix: pretty print expected aswell when format is "pretty"

flake.nix

@@ -99,6 +99,13 @@
                   };
                 };
               }
+              {
+                name = "pretty-unit";
+                format = "pretty";
+                pos = __curPos;
+                expected = pkgs.hello;
+                actual = pkgs.hello;
+              }
               {
                 name = "test-drv";
                 pos = __curPos;