feat(xatu): check let binding order

src/xatu/testdata/clean.nix

@@ -1,6 +1,22 @@
 { pkgs }:
 
-{
+let
+  first = 1;
+  second = first;
+in {
+  firstField = first;
+  secondField = second;
+  nested = let
+    innerSecond = innerFirst;
+    innerFirst = first;
+  in {
+    result = innerSecond;
+  };
+  parameter = first: let
+    usesParameter = first;
+  in {
+    result = usesParameter;
+  };
   path = ./. + "/foo";
   url = "https://example.com";
   inherit pkgs;