test(xatu): add let ordering fixture

src/xatu/BUILD

@@ -34,6 +34,7 @@ rust_test(
     data = [
         "testdata/clean.nix",
         "testdata/invalid.nix",
+        "testdata/let_order.nix",
         "testdata/lint_findings.nix",
     ],
     deps = [

src/xatu/testdata/let_order.nix

@@ -0,0 +1,7 @@
+let
+  second = 2;
+  first = 1;
+in {
+  firstField = first;
+  secondField = second;
+}

src/xatu/xatu_test.rs

@@ -43,12 +43,7 @@ fn xatu_rejects_invalid_source() -> googletest::Result<()> {
 
 #[gtest]
 fn xatu_reports_out_of_order_let_binding() -> googletest::Result<()> {
-    let report = xatu::future_sight(vec![SourceFile {
-        path: PathBuf::from("test.nix"),
-        source: "let\n  second = 2;\n  first = 1;\nin {\n  firstField = first;\n  secondField = second;\n}\n"
-            .to_owned(),
-    }])
-    .or_fail()?;
+    let report = xatu::future_sight(vec![fixture_source("testdata/let_order.nix")?]).or_fail()?;
 
     expect_that!(
         report